Invasive pulmonary aspergillosis in the ICU: the corticosteroid link

Abstract

Invasive pulmonary aspergillosis (IPA) is a life-threatening fungal infection traditionally associated with severely immunocompromised hosts, particularly those with hematologic malignancies. However, its epidemiological profile has shifted in recent years, with a rising incidence among critically ill patients in intensive care units (ICUs), many of whom lack classical risk factors. This change is driven by increased use of corticosteroids and immunomodulatory therapies, the growing prevalence of chronic lung disease, and severe viral pneumonias such as influenza and COVID-19. In these patients, airway epithelial injury, immune dysregulation, and mechanical ventilation facilitate fungal invasion even in the absence of profound immunosuppression. Corticosteroids play a central role in IPA pathogenesis. While they limit hyperinflammation, they simultaneously impair fungal clearance by suppressing NF-κB signaling, downregulating TNF-α production, and promoting IL-10 secretion, resulting in a Th2-skewed immune profile. Neutrophil recruitment persists but becomes dysregulated, contributing to tissue injury rather than effective pathogen elimination. Corticosteroids may also directly enhance Aspergillus growth, further compounding risk. Diagnosis of IPA in ICU patients remain challenging because radiological hallmarks such as the halo sign are uncommon, and distinguishing colonization from invasive disease is difficult. Serum and bronchoalveolar lavage galactomannan, β-D-glucan assays, and PCR can improve early detection, but no single test is definitive in this heterogeneous population. As much as possible, high-quality lower respiratory tract samples should be obtained. Furthermore, effective treatment requires not only timely diagnosis, but also careful selection of antifungal taking into consideration pharmacologic challenges of ICU patients and pharmacodynamics of antifungals. Recognition of high-risk patients such as those receiving corticosteroids, those with chronic lung disease, severe viral pneumonia, or requiring invasive ventilation is critical to improve outcomes. Mortality in this group can exceed that of neutropenic patients, underscoring the need for heightened clinical suspicion and timely antifungal therapy. A deeper understanding of the immunopathogenesis of IPA in non-neutropenic patients, particularly the dual effects of corticosteroids on inflammation and host defense, may inform risk stratification and guide earlier intervention. Enhanced surveillance, prompt diagnostic workup, and judicious use of immunomodulatory therapy represent key strategies to mitigate the rising burden of this devastating infection in ICU settings.

Introduction

Invasive pulmonary aspergillosis (IPA) is a life-threatening fungal infection and the leading mycosis affecting severely immunocompromised individuals, particularly patients with hematologic malignancies. The global burden of IPA is likely underestimated, with current estimates exceeding 300,000 cases annually [1]. Although Aspergillus fumigatus remains the most commonly identified species, non-fumigatus species, such as A. flavus, A. niger, and A. terreus, now account for approximately 44.5% of infections [2]. Rising antifungal resistance and breakthrough infections further complicate clinical management and highlight the need for enhanced surveillance and early diagnosis [3].

The epidemiology of invasive fungal infections has shifted significantly in recent years, with growing proportion of IPA reported among critically ill patients admitted to intensive care units (ICUs), many without classic risk factors such as neutropenia [4]. In a retrospective cohort from the Hospital Clínic of Barcelona, Spain (2017–2021), the overall incidence of invasive fungal infections (IFIs) was 24.3 cases per 10,000 admissions, and IPA ranked second in frequency after candidemia. Notably, 54% of IFIs were diagnosed in the ICU, while only 25% occurred in neutropenic patients. Corticosteroid use was documented in 62% of cases, viral coinfection in 47%, and 16% of episodes were classified as breakthrough infections [2]. These findings underscore the increasing complexity of patient profiles.

The true incidence of IPA in ICU settings remains unclear. However, it is estimated that approximately 1 in every 27 ICU patients will develop invasive aspergillosis (IA) during their stay [5]. Autopsy studies have identified aspergillosis as the most frequently missed infectious diagnosis among ICU deaths [6]. The increasing number of IA cases in the ICU may indicate greater clinical awareness together with an expanding population of patients exposed to corticosteroids and other immunomodulatory therapies.

Given the high mortality of IPA, which can reach up to 80%, [5, 7] early identification of patients at risk is essential. This review examines risk factors for IPA in critically ill patients, with particular emphasis on the role of corticosteroids, and explore the underlying pathogenesis, diagnostic and treatment challenges specific to this population. Our goal is to support earlier recognition and timely management of this life-threatening fungal infection.

Risk factors for IPA in the ICU

Patients in ICUs and those with severe immunosuppression exhibit distinct IPA risk profiles [4]. Prolonged neutropenia and post-transplant immunosuppression remain the most recognized risk factors in patients with hematologic malignancies [8, 9]. In contrast, in critically ill ICU populations, risk is driven mainly by corticosteroid therapy (particularly doses >20 mg/day of prednisone or equivalent) used for chronic obstructive pulmonary disease (COPD), sepsis, or acute respiratory distress syndrome (ARDS), and by severe viral infections [10, 11].

It is important to understand that the occurrence of IPA in apparently immunocompetent ICU patients may reflect the biphasic immune response to sepsis. This response involves an initial hyperinflammatory phase followed, or occurring concurrently, by immunoparalysis, characterized by impaired function of both innate and adaptive immune cells. Monocytes and neutrophils become hypofunctional, lymphocyte apoptosis leads to lymphopenia, and expansion of regulatory T cells exacerbates immunosuppression by further attenuating effector responses [12, 13].

Comorbidities such as COPD also contribute to IPA risk, likely due to the combined effects of corticosteroid exposure and structural lung abnormalities such as bronchiectasis, which facilitate Aspergillus colonization. The steroid dose associated with IPA risk may vary by underlying condition; in chronic lung disease, IPA can develop even at lower corticosteroid doses [14]. In Spain, an estimated 10–20% of COPD patients receiving corticosteroids develop IPA, and mortality in these group exceeds that of COPD patients without IPA and may even surpass the mortality observed in neutropenic individuals [15]. Among patients with COPD, the accumulated dose of corticosteroids (>700 mg) in the 3 months before admission (Odds Ratio [OR] 2.987; 95% CI [1.263–7.060]; p = 0.013) and during admission associate (4.068 [2.022–10.324], p < 0.001) with an increased risk of IPA [16].

Severe viral pneumonias, particularly due to influenza or Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2), have emerged as significant risk factors for IPA in ICU settings. In a cohort of 432 ICU patients with influenza (2009–2016), 19% developed influenza-associated pulmonary aspergillosis (IAPA), with significantly higher ICU mortality than those without IPA (45% vs. 20%). IPA was independently associated with both influenza infection (adjusted odds ratio [aOR] 5.19; 95% CI 2.63–10.26; p < 0.0001) and corticosteroid exposure within the four weeks prior to ICU admission (aOR 1.84; 95% CI 1.14–2.98; p = 0.013). The incidence of IAPA was similar between Influenza A and B, and remained above 10% across all calendar years, suggesting that viral subtype may not play a critical role [17]. Notably, influenza-related ARDS often leads to corticosteroid administration, further increasing IPA risk.

Coronavirus Disease (COVID)-19–associated pulmonary aspergillosis (CAPA) has emerged as a serious complication in critically ill patients, with risk factors including the combined use of dexamethasone and tocilizumab, invasive mechanical ventilation, and advanced age [18–20]. In a study of 186 patients with CAPA, half of them received corticosteroids [19]. Despite antifungal treatment, approximately 20% of patients with severe influenza or COVID-19 pneumonia who require invasive ventilation develop IPA, and mortality remains high [21]. Notably, the incidence of IPA is lower in COVID-19 than in influenza [22].

Patients with solid tumors, particularly those with primary lung cancer or pulmonary metastatic dissemination, are also increasingly recognized as being at risk for IPA [23]. Many of these patients present with additional predisposing factors such as corticosteroid therapy, viral coinfections, and underlying chronic lung disease, risk factors shared across other solid malignancies [2]. Furthermore, targeted therapies such as Bruton tyrosine kinase (BTK), Janus kinase (JAK), or phosphoinositide 3-kinase (PI3K) inhibitors may increase susceptibility to IPA due to off-target immunosuppressive effects [24, 25]. Patients with hepatic dysfunction are also at increased risk, due to impaired phagocytosis, defective chemotaxis, reduced complement activity, and suboptimal opsonization [26].

A specific clinical form of IPA, known as Aspergillus tracheobronchitis, involves focal invasion of the tracheobronchial tree. While it shares similar risk factors with classic IPA, it is associated with even higher mortality rates [27].

Pathophysiological and immunological basis of IPA

Aspergillus spp. are ubiquitous environmental fungi with efficient airborne dispersal and rapid conidial germination. Inhalation of conidia represents the primary route of entry into the host [28]. Their small diameter (2–3 μm) enables them to bypass upper airway defenses and deposit in the alveoli. In immunocompetent hosts, innate immune mechanisms efficiently eliminate them. However, in susceptible hosts, impaired clearance allows germination and invasive growth. Thus, clinical outcome is determined by the interplay between fungal attributes and host immune competence.

The respiratory epithelium constitutes the initial barrier against fungal invasion, acting both as a physical shield and as a source of immune mediators [29]. This barrier is frequently compromised in critically ill patients, especially due to mechanical ventilation, severe viral infections, or immune alterations, facilitating fungal persistence and tissue invasion. A defining feature of invasive disease is angioinvasion, in which Aspergillus breaches the epithelium and invades blood vessels, causing vascular occlusion, tissue necrosis, and impaired antifungal delivery. Aspergillus can also suppress local angiogenesis, promoting infection persistence and delays in therapeutic response. This angioinvasive behavior is typically seen in neutropenic patients, whereas it may occur more gradually in non-neutropenic individuals.

Fungal elements trigger innate immune activation at the airway epithelium, initiating cytokine responses such as interferon-β (IFN-β) secretion. This promotes the recruitment of circulating immune cells (monocytes, neutrophils, and dendritic cells) to the infected site. Macrophages and monocytes play a pivotal role by engulfing and neutralizing conidia, preventing their transformation into hyphae. Neutrophils are critical for eliminating germinating hyphae through oxidative mechanisms, while dendritic cells link innate and adaptive immunity by presenting fungal antigens to T cells [30–32].

Recognition of Aspergillus by the immune system depends on pattern recognition receptors (PRRs) expressed by innate immune cells, such as dendritic cells and myeloid cells (macrophages, monocytes, and neutrophils). These receptors detect conserved fungal structures termed pathogen-associated molecular patterns (PAMPs). Among the most relevant PRRs in aspergillosis are Dectin-1, DC-SIGN (dendritic cell-specific intercellular adhesion molecule-3-grabbing non-integrin), pentraxin 3 (PTX3) and Toll-like receptors (TLRs), particularly TLR2, TLR4. and TLR9 [33, 34].

The activation of TLR4 in dendritic cells and macrophages induce the production of IL-12p70, TNF-α, and IL-1β, thereby promoting T helper type 1 (Th1) polarization, M1 macrophage activation, and the secretion of IFN-γ, which amplifies the inflammatory response and enhances pathogen elimination [35–39]. Furthermore, TLR4-mediated signalling in these antigen-presenting cells promotes the expression of costimulatory molecules (such as CD86) and the production of IL-12, both of which are essential for Th1 lymphocyte differentiation and NK cell activation [37]. In contrast, TLR2 activation drives a Th2-biased response dominated by IL-4 and IL-10, which has been linked to increased susceptibility and allergic inflammation [40]. A balanced Th1/Th2 response is essential for optimal host protection [41].

TLR9 plays a pivotal role in innate defense against A. fumigatus by recognizing unmethylated CpG DNA motifs within the fungal genome, thereby triggering immune cell activation and pro-inflammatory cytokine production [42–44]. TLR9 activation occurs primarily in endosomal compartments following phagocytosis of Aspergillus conidia by macrophages and dendritic cells, facilitating phagosome maturation and antifungal inflammatory responses. TLR9 signalling induces cytokines such as TNF-α and IL-6, promoting neutrophil recruitment and effector cell activation [44, 45]. In addition, TLR9 modulates the expression of other antifungal receptors, including dectin-1, suggesting a coordinating role in innate immunity. In murine models, TLR9 deficiency is associated with impaired inflammatory responses and increased susceptibility to pulmonary Aspergillus infection, particularly under immunodeficient or fungal asthma conditions [46, 47].

Dectin-1 specifically recognizes β-D-glucan and galactomannan, which are key constituents of the fungal cell wall. This recognition triggers a cascade of immune responses, including phagocytosis, generation of reactive oxygen species via Nicotinamide Adenine Dinucleotide Phosphate (NADPH) oxidase, and pro-inflammatory cytokine release. Macrophages eliminate fungal elements through oxidative and lysosomal mechanisms, whereas neutrophils rely more heavily on oxidative pathways [48]. Inherited defects in NADPH oxidase function, such as those causing chronic granulomatous disease, confer marked susceptibility to invasive aspergillosis [49].

Soluble PRRs such as pentraxin-3 (PTX3), released by macrophages and epithelial cells, further enhance fungal recognition. PTX3 binds to fungal components like galactomannan and opsonizes conidia, thereby facilitating their uptake by neutrophils. This mechanism is considered critical in the early containment of infection [50].

Finally, while the exact contribution of humoral immunity remains uncertain, Aspergillus-specific T cell responses, particularly through CD4 + T-helper cells, is essential to host defense against fungal infection and establishing immunologic memory. CD4⁺ T lymphocytes provide the main protective immunity, with Th1 cells (producers of IFN-γ) associated with effective antifungal responses and improved clinical outcomes in invasive aspergillosis, whereas Th2-dominated responses are linked to allergic manifestations such as allergic bronchopulmonary aspergillosis and may exacerbate disease [45, 51–53]. Th17 cells also contribute by promoting neutrophil recruitment, activation, and antimicrobial peptide production, although their role can be dual, supporting protection or driving pathological inflammation depending on the context [45, 54, 55]. Regulatory T cells (Tregs) modulate immunity by suppressing inflammation and controlling Th17 and Th2 expansion, thereby preventing tissue damage and allergic responses. [45, 55, 56]

Overall, the presence and expansion of Aspergillus-specific T cells, particularly those with a Th1 phenotype, correlates with infection resolution and may serve as a basis for monitoring and immunotherapeutic strategies. [52, 58–59] Pathophysiological mechanisms of IPA are summarized in Table 1.

Table 1.

Pathophysiological mechanisms involved in invasive pulmonary aspergillosis

Pathway	Mechanism	Impact
Respiratory epithelium	First barrier. Secretes IFN-β to recruit monocytes, neutrophils, and dendritic cells.	Impaired fungal clearance in ventilated patients and lung structural damage (e.g. viral infections).
Innate immune cells	Engulf conidia (macrophages) and kill hyphae (neutrophils) via oxidative burst (NADPH oxidase) and connect innate with adaptative immune response (dendritic cells).	Immunological defects (e.g. chronic granulomatous disease, corticosteroid use) lead to invasive disease.
TLR4	Induces TNF-α, IFN-γ and IL-12 production. Macrophage activation.	Promotes protective inflammation by driving Th1 polarization.
TLR2	Activates IL-4 and IL-10 secretion.	Allergic inflammation by promoting Th2 response.
TLR9	Induces TNF-α and IL-6. Promotes neutrophil recruitment. Modulates expression of Dectin-1.	Deficiency leads to reduced inflammatory response.
Dectin-1	Binds β-D-glucan and galactomannan on fungal cell walls.	Essential for antifungal defense; mutations cause susceptibility to IPA.
Pentraxin-3	Released by macrophages and epithelial cells. Opsonize conidia and enhances fungal recognition by neutrophils.	Deficiency linked to poor fungal clearance and severe IPA.
Th1 cells	Produce IFN-γ Promote macrophage activations.	Correlate with infection resolution.
Th2 cells	Linked to allergic response.	Associate with allergic bronchopulmonary aspergillosis.
Th17 cells	Neutrophil recruitment and activation.	If overactive, produces inflammation.
Regulatory T cells (Tregs)	Suppress excessive inflammation by suppressing Th2 and Th17 expansion.	Prevent tissue damage and allergic responses.

Role of corticosteroids

Experimental studies comparing murine models treated with chemotherapy versus corticosteroids have demonstrated that the pathogenesis of IPA varies depending on the immunosuppressive regimen. While chemotherapy primarily facilitates fungal proliferation, corticosteroid use is associated with a deleterious host-driven immune response [60].

Under corticosteroid therapy, innate immune components remain present but are functionally impaired [61]. Although fungal growth may be initially limited due to partial inhibition of conidial germination, the host’s capacity to clear the pathogen is significantly compromised. Corticosteroids suppress the production of tumor necrosis factor-alpha (TNF-α), a cytokine essential for the recruitment of polymorphonuclear neutrophils (PMNs) and downregulate NF-κB signalling, a central inflammatory pathway, thereby weakening the antifungal response. However, interleukin-8 (IL-8) activity remains unaffected, leading to excessive PMN recruitment, which contributes to lung injury through intense neutrophilic infiltration and the formation of pulmonary lesions rather than effective fungal clearance. Additionally, corticosteroids increase the production of the anti-inflammatory cytokine IL-10, further reducing protective immune responses without mitigating tissue-damaging inflammation.

Paradoxically, corticosteroid treatment enhances the inflammatory response, and excessive PMN accumulation can result in respiratory deterioration and tissue damage. While inflammation is essential for fungal containment, its uncontrolled activation can be fatal. Beyond impairing host immunity, corticosteroids have also been shown to directly accelerate Aspergillus growth [62].

Chronic obstructive pulmonary diseases (COPD)

Patients with structural lung disease are at increased risk of infections by various pathogens, including Aspergillus. In particular, IPA occurs more frequently in individuals with chronic obstructive pulmonary disease (COPD), who are often colonized by Aspergillus spp. Impaired mucociliary clearance and repeated damage to the bronchial epithelium, due to tobacco exposure and recurrent infections, create a permissive environment for fungal adhesion and invasion. Notably, approximately 77% of COPD patients are already receiving corticosteroids at hospital admission, a proportion that rises to 96% during hospitalization [63]. Corticosteroids enhance fungal growth by impairing antifungal immune mechanisms, including inhibition of reactive oxygen species crucial for conidial clearance by alveolar macrophages. They also suppress neutrophil function, downregulate the Th1-mediated immune response, and promote a Th2-skewed profile, thereby further compromising antifungal host defenses [15].

Viral infections

Viral infections such as influenza and SARS-CoV-2 have been increasingly recognized as significant risk factors for IPA [19–21, 64]. Influenza causes both bronchial and alveolar epithelial damage along with mucociliary dysfunction, facilitating fungal invasion. The virus also disrupts local and systemic cytokine responses, impairing phagocytic activity and adaptive immune mechanisms [65]. Proinflammatory reactions lead to local tissue injury and immune dysregulation within the alveoli. Respiratory viruses degrade the mucosal barrier, induce immune paralysis, and upregulate IL-10 promoting a shift from a protective Th1 response to a permissive Th2 profile, which facilitates fungal invasion [30]. Moreover, influenza-associated ARDS and hypoxia can exacerbate immune suppression.

In CAPA, the pathogenesis appears to differ. CAPA is believed to result from persistent activation of the innate immune system, leading to a state of hyperinflammation [22]. Compared to influenza-associated IPA, CAPA tends to present later in the disease course, as fungal infection may initially remain confined to the airways before progressing to angioinvasion. Its immunopathogenesis involves a combination of viral-induced epithelial damage, fungal virulence factors, and host immune dysregulation. SARS-CoV-2 infection injures airway epithelial cells, disrupting tight junctions and impairing mucociliary clearance. Damaged epithelial cells express integrins that facilitate Aspergillus adhesion, further promoted by fungal cell wall components. Infected epithelial cells also release danger-associated molecular patterns (DAMPs), which stimulate surrounding cells and alveolar macrophages, triggering a proinflammatory cytokine cascade. Hyperinflammation, a hallmark of severe COVID-19, thus promotes susceptibility to fungal superinfection. Additionally, COVID-19 induces profound lymphopenia and T-cell dysfunction, partly due to impaired type I interferon responses, which are essential for dendritic cell priming and Th1 activation against Aspergillus. This results in impaired cellular immunity and increased vulnerability to invasive fungal disease.

Diagnosis

The diagnosis of IPA in critically ill patients remains particularly challenging. These difficulties are largely due to the heterogeneity of the ICU population [66].

A key challenge lies in distinguishing colonization from true infection. Although several studies have aimed to identify factors that distinguish colonization from true infection, [67–69] the authors believe that it is difficult to confidently classify Aspergillus isolation as mere colonization in ICU patients. This perspective is grounded in the pathophysiological understanding of fungal infections: any pulmonary dysfunction, such as that caused by mechanical ventilation, significant comorbidities, or immunosuppression, may allow a single Aspergillus spore to trigger a severe infection in an already critically ill host. We therefore recommend that in intubated ICU patients, or those with pulmonary or immunological dysfunction, a positive Aspergillus culture should prompt early antifungal treatment. Subsequent de-escalation may be considered if the patient’s clinical course is favourable and predisposing factors for invasive infection have resolved. Importantly, negative sputum cultures do not exclude the diagnosis of IPA, and more sensitive methods such as bronchoalveolar lavage cultures, galactomannan testing, or PCR may be necessary to support the diagnosis [70].

Signs and symptoms of angioinvasion are typically absent in immunocompetent individuals, which may explain the lower frequency of characteristic radiological findings in this population. While chest computed tomography (CT) features such as multiple nodules and the halo sign are typical in neutropenic patients with IPA, the sensitivity of the halo sign drops substantially in non-neutropenic individuals. It has been reported in only 24% of patients without hematologic malignancy, compared with 82% of those with neutropenic hematologic malignancies [71]. In patients with preserved neutrophil counts, the halo sign is rare and its absence lacks negative predictive value. Instead, findings such as nonspecific pneumonia and cavitary infiltrates (Fig. 1) are more frequently observed in those with underlying lung disease [14].

Fig. 1.

CT-scan of a patient with Chronic Obstructive Pulmonary Disease (COPD) who developed Invasive Pulmonary Aspergillosis

In critically ill patients, obtaining high-quality respiratory samples is essential for accurate microbiological diagnosis. Fiberoptic bronchoscopy (FOB) with bronchoalveolar lavage (BAL) is considered the most effective diagnostic approach. FOB enables direct visualization of airway abnormalities, while BAL improves the likelihood of identifying the causative pathogen by collecting samples from the lower respiratory tract. This allows for a more precise and specific diagnosis using both culture and non-culture techniques. When performed by experienced personnel, the procedure is safe, though it requires trained staff and specialized equipment. Tracheal aspiration represents a less invasive alternative that is simple, safe, cost-effective, and does not require specialized expertise. However, because it samples the upper respiratory tract, distinguishing between true infection and colonization can be challenging. An intermediate option is the bronchoalveolar mini lavage, which is also less invasive but has the disadvantage of being a blind procedure, as the sampling site cannot be precisely directed. Sputum cultures are not considered reliable because they are often of poor quality. Since Aspergillus species are ubiquitous in the environment, positive results may reflect colonization or contamination rather than true infection [72].

Microbiological testing in non-hematologic patients has shown limited sensitivity and specificity [10]. Table 2 provides an overview of sensitivity and specificity of microbiological tests. Direct microscopy with fluorescent dyes (e.g., Calcofluor) have the advantage of high sensitivity, rapid turnaround time, and broad applicability. The main disadvantage is the inability to distinguish between Aspergillus spp and other filamentous fungi [73]. Bronchoalveolar lavage (BAL) alone has low sensitivity (up to 50%), but it provides the advantage to identify the species and identify azole resistance by using MALDI-TOF mass spectrometry, species-specific nucleic acid amplification methods or sequencing methods [74]. Serum (1→3)-β-D-glucan (BDG) is considered highly sensitive and specific for invasive fungal infections, and its positive and negative predictive values are comparable to those of galactomannan in the diagnosis of IPA [75, 76]. It is not recommended to perform BDG in BAL; it has low specificity and it is associated with false positive results. Galactomannan (GM) testing may be similarly useful in patients without hematologic malignancies as in those with them. Detection of galactomannan in serum or BAL may enable earlier diagnosis, as levels can rise before clinical, radiological, or culture-based signs become evident. Among neutropenic patients, BAL galactomannan demonstrates higher sensitivity and similar specificity when compared to serum testing [76]. However, because GM is cleared by neutrophils, its diagnostic utility is reduced in non-neutropenic patients, making respiratory samples generally preferable in this population [5, 77].

Table 2.

Sensitivity and specificity of microbiology tests

Sample type	Diagnostic test	Study	Sensitivity	Specificity
Blood	Beta-D-glucan	Pazos C, et al.76	87.5%	89.6%
Blood	Galactomannan	Pfeiffer CD, et al.96	88–90%	68–74%
BAL	Galactomannan	He H, et al.97	88.9%	100%
BAL	Culture	Meersseman W, et al.10	Up to 50%	20–70%
Blood	PCR	Cruciani M, et al. 98	71-85.5%	70-86.6%
BAL	PCR	Rath PM, et al. 99	68–94%	80–98%

Polymerase chain reaction (PCR) testing on bronchoalveolar lavage (BAL) or serum allows for the detection of Aspergillus DNA [78, 79]. Its routine use remains limited due to availability constraints and the potential for false-positive results. Rapid and accurate diagnostics are essential to guide timely antifungal treatment, yet, like culture alone, PCR alone cannot reliably differentiate colonization from true invasive infection. Therefore, a combination of diagnostic approaches is recommended to improve accuracy and clinical decision-making. Aspergillus PCR from blood and BAL show modest sensitivity and specificity, but they increase when Aspergillus PCR in BAL is combined with BAL galactomannan [80].

Treatment

Both voriconazole and isavuconazole are currently recommended as first-line treatments for invasive pulmonary aspergillosis [81].

Although voriconazole (VCZ) remains the standard first-line treatment for invasive pulmonary aspergillosis, [82] its use in critically ill patients presents several challenges. First, VCZ metabolism is not fully understood. It inhibits multiple cytochrome P450 enzymes, and it has been associated with elevated hepatic transaminases, phototoxicity, neurological adverse effects, and QTc prolongation [83]. Second, VCZ shows considerable intra and inter patient pharmacokinetic variability, demanding close therapeutic drug monitoring (TDM). And third, VCZ is frequently associated with drug–drug interactions, which is particularly problematic in ICU patients receiving multiple medications [84].

Isavuconazole (ISA) represents an alternative first-line agent with a more predictable pharmacokinetic profile, fewer toxicities, and fewer clinically significant drug–drug interactions, making its use more attractive in critically ill patients [85]. Although not entirely free from drug–drug interactions since it is metabolized by CYP3A4, its drug-drug interaction activity is considerably weaker than that of other azoles, representing one of the key advantages of this antifungal agent [86]. Furthermore, ISA also provides broad-spectrum coverage, being active against both aspergillosis and mucormycosis [87].

Liposomal amphotericin B (L-AmB) can serve as an alternative first-line option for patients who cannot tolerate azoles, have hepatic impairment, or when azole resistance or clinically significant drug–drug interactions are anticipated. Although L-AmB is associated with nephrotoxicity, this effect is usually reversible [88].

Second-line options include posaconazole (PCZ) and echinocandins. Notably, posaconazole has been associated with breakthrough infections during prophylactic use and reduced therapeutic efficacy when subtherapeutic concentrations occur [89]. Echinocandins should not be administered as monotherapy and should only be used as salvage therapy in combination with first-line antifungals [90].

For therapeutic drug monitoring (TDM), the first sample (trough sample) should be obtained once the steady state has been reached, typically 3–7 days after initiating therapy (depending on the antifungal), and then repeated at least once per week after the dose has stabilized. For voriconazole (VCZ), there is a well-established correlation between serum VCZ concentrations and clinical efficacy [91]. The therapeutic range is 1–6 mg/L, with toxicity usually observed at concentrations >6 mg/L. Isavuconazoles (ISA) currently does not require TDM because of its linear pharmokinetics [92]. However, if monitoring is performed, it should be done 5–7 days after treatment initiation because ISA has a long half-life. A serum concentration >1 mg/L is generally recommended, and toxicity may occur at levels >4.8 mg/L. For posaconazole (PCZ), drug concentrations are often higher in bronchoalveolar lavage fluid than in plasma, although not clear correlation has been defined. The recommended therapeutic concentration is >1 mg/L, with toxicity reported at 2–4 mg/L. TDM for PCZ is advised 5–7 days after therapy initiation. TDM is not required for amphotericin B formulations nor echinocandins.

In a large international study evaluating the probability that a given antifungal dosing regimen achieves the desired pharmacodynamic target (target attainment) among critically ill patients, isavuconazole showed the best results, with 60% and 100% of patients reaching target attainment at days 1–3 and 4–7, respectively. In contrast, voriconazole, posaconazole, micafungin, and L-AmB demonstrated the lowest rates of target attainment, suggesting that current dosing regimens may not provide adequate antifungal exposure for effective treatment of fungal infections [93].

Antifungal combination therapy is not generally recommended, but it could be considered in selected cases, such as suspected resistance or risk of coinfection. Potential benefits include a broad antifungal spectrum with antifungal synergism [94]. The most common combination is a first-line antifungal with an echinocandin [90, 95].

Administration of inhaled antifungal therapy can be complex and technically challenging but may have a role in Aspergillus tracheobronchitis. In patients with a pulmonary infiltrate, amphotericin B is preferred, whereas in patients without infiltrates but with a positive galactomannan, an echinocandin is recommended.

Conclusion

The development of IPA in the ICU is closely linked to immune system dysfunction or underlying pulmonary alterations due to comorbidities or viral infections. Corticosteroids play a central role in increasing the risk of infection in these patients, not only by suppressing the host immune response but also by directly promoting fungal growth. Identifying patients at risk is therefore essential in the management of critically ill individuals. ICU physicians should maintain a high index of suspicion by recognizing relevant host factors and understanding the underlying pathophysiology of the disease, both of which are critical for early diagnosis, prompt initiation of treatment, monitorization and improved clinical outcomes.

Abbreviations

IPA

Invasive Pulmonary Aspergillosis

ICU

Intensive Care Unit

COVID-19

Coronavirus Disease 2019

NF-κB

Nuclear Factor kappa-light-chain-enhancer of activated B cells

TNF-α

Tumor Necrosis Factor alpha

IL

Interleukin

Th1

T helper type 1

Th2

T helper type 2

CT

Computed Tomography

PCR

Polymerase Chain Reaction

IFI

Invasive Fungal Infection

IA

Invasive Aspergillosis

COPD

Chronic Obstructive Pulmonary Disease

ARDS

Acute Respiratory Distress Syndrome

SARS-CoV-2

Severe Acute Respiratory Syndrome Coronavirus 2

IAPA

Influenza-Associated Pulmonary Aspergillosis

CAPA

COVID-19–Associated Pulmonary Aspergillosis

BTK

Bruton Tyrosine Kinase

JAK

Janus Kinase

PI3K

Phosphoinositide 3-Kinase

PRRs

Pattern Recognition Receptors

PAMPs

Pathogen-Associated Molecular Patterns

DC-SIGN

Dendritic Cell-Specific Intercellular Adhesion Molecule-3-Grabbing Non-Integrin

PTX3

Pentraxin 3

TLRs

Toll-Like Receptors

IL

Interleukin

NADPH

Nicotinamide Adenine Dinucleotide Phosphate

TNF

Tumor Necrosis Factor

DAMPs

Danger-Associated Molecular Patterns

GM

Galactomannan

FOB

Fiberoptic bronchoscopy

BAL

Bronchoalveolar Lavage

DNA

Deoxyribonucleic Acid

BDG

(1→3)-β-D-glucan

VCZ

Voriconazole

ISA

Isavuconazole

PCZ

Posaconazole

L-Amb

Liposomal amphotericin B

TDM

Therapeutic drug monitoring

Author contributions

Conception and design: CT-C, CG-V. Manuscript writing: CT-C, CG-V. Critical review and revision of manuscript: AV, AG, NE. Final approval of manuscript: all authors.

Funding

This work was co-funded by a research grant (SGR 01324 Q5856414G) from the AGAUR (Agencia de Gestión de Ayudas Universitarias y de Investigación) of Catalunya. C.G.-V. [PI24/01776 and ICI21/00103] has received research grants funded by Institut de Salut Carlos III (ISCIII) and co-funded by the European Union - Next GenerationEU, which finance the actions of the Recovery and Resilience Facility (RRF/RTRP). The funders had neither a specific role in study design or collection of data nor in writing of the paper or decision to submit.

Data availability

No datasets were generated or analysed during the current study.

Declarations

Ethics approval and consent to participate

Not applicable.

Consent for publication

Not applicable.

Competing interests

CG-V has received honoraria for talks on behalf of Gilead Science, MSD, Pfizer, Shionogi, AbbVie, Advanz Pharma, and a grant support from Gilead Science, and Mudipharma. AS has received honoraria for talks and advisory boards from Merck Sharp and Dohme, Pfizer, Shionogi, Novartis, Angelini and Menarini as well as grant support from Pfizer and Gilead Sciences.