Table 3.

Advantages and disadvantages of existing diagnostic methods for paediatric pneumonia

	Description	Advantages	Disadvantages
Field assessment methods
WHO case definition	Acute (≤ 14 days) non-severe pneumonia: cough or difficulty breathing with tachypnoea or lower chest wall indrawing; severe pneumonia: pneumonia plus any general danger sign. Another definition of severe pneumonia that follows the guidelines is acute (≤14 days) episode of cough or difficulty breathing with either a general danger sign or hypoxaemia.30	High sensitivity, easy to implement in resource-poor settings; provides a standardised approach that is comparable with other studies.	Low specificity, fails to distinguish between bacterial and viral causes,21 can lead to non-differential misclassification of outcome, danger signs are subjective and difficult to identify and the diagnosis of chest indrawing requires standardised training.
Physician diagnosis of pneumonia	Integrates clinical knowledge, evidence-based knowledge and local practices for the identification of pneumonia. Chest indrawing and general signs of respiratory distress and general danger signs.	Follows common clinical guidelines and practices, benefits from an expert diagnosis.	Highly subjective to inter-observer heterogeneity, cannot be replicated, might need an adjudication panel for consensus.
Respiratory rate	Number of breaths taken per min. If higher than normal (defined by pre-specified cut-offs), considered to be tachypnoea.	WHO cutoffs are age-specific: ≥60 breaths per min for children <2 months of age, ≥50 breaths per min for children between 2 and 12 months of age, and ≥40 breaths per min for children between 12 and 59 months of age.46	Difficult to standardise measurement and inadequate reference ranges for a variety of settings (ie, high altitude).
Ancillary diagnostic methods
Arterial oxyhaemoglobin saturation	Measured using pulse oximeters to determine hypoxemia (typically a SpO2 measurement of <90%) that occurs in pneumonia because of ventilation-perfusion mismatch in the lungs.53	Inexpensive, portable, and, reliable in a variety of settings.52 Provides objective diagnostic criteria that is highly specific when combined with respiratory signs and symptoms, and a well recognised indicator of pneumonia severity and mortality in children.46	Unknown utility in home-based surveillance studies, reference values for SpO2 in healthy children are not well established (especially at varying altitudes), low sensitivity, does not provide any information or indication about the causes of pneumonia.
Chest auscultation	Inspiratory lung crackles represent the equalisation of distal airway pressures caused by the abrupt opening of collapsed alveoli and adjacent airways.	Likelihood of radiographic pneumonia increases with crackles44, 61 and provides objective diagnostic criteria when done correctly.	Difficult to achieve reliable, reproducible interpretations of lung sounds, requires specialised training, quiet examination areas (especially with children), does not provide any information about the causes of pneumonia.
Host-response biomarker testing	Used to guide therapy based on the cause, or causes, of pneumonia. Examples include CRP, procalcitonin, chitinase 3-like-1, haptoglobin, tumour necrosis factor receptor 2 or IL-10, and tissue inhibitor of metalloproteinases 1.	Might identify children who do not require antibiotic treatment, past studies have shown high sensitivity and accuracy.68, 69, 94	Hard to implement in resource-poor settings, point-of-care tests do not exist, not all bacterial infections affect host-response biomarkers, host-response might be affected by malnutrition and immunosuppression.
Aetiology detection	Ascertained in sputum, blood, urine, nasal, nasopharyngeal or oropharyngeal swab, nasal or nasopharyngeal wash, and by nasopharyngeal, lung, or pleural fluid aspirates using microscopy, standard microbiological cultures, serology, antigen detection, or molecular methods.	Can provide confirmation of bacterial or viral causes of pneumonia, and identify mixed causes.81 This method might also help to reduce heterogeneity in pneumonia phenotypes by identifying if the underlying infection is predominantly bacterial, viral, or instances where there are a mixture of causes.	Reliable findings require standardised sample types, approaches for sample collection, and diagnostic methods. Different sample types have varying sensitivities for different pathogens. Some (such as pleural fluid) are invasive to collect.
Chest radiography	Visualises consolidation and interstitial patterns.	Widely available, well developed standard for image interpretation. Historically, this method is the gold standard for diagnosing pneumonia.66 Allows for comparisons between countries, regions, and time periods.	Exposure to radiation, not all cases show evidence of consolidation or interstitial patterns (particularly early in disease). Requires standardised training and maintaining study staff to reliably interpret, and quality control to mitigate high inter-reader variability. Expensive equipment requires power supply.
Lung ultrasound	Visualises consolidation and interstitial patterns.	Rapid, point-of-care diagnostic test without radiation,94 and strong diagnostic validity shown in various age groups.92, 93	Further validation needed in large studies. Not all cases show evidence of consolidation or interstitial patterns (particularly early in disease or in instances of malnourishment). Requires standardised training and maintaining study staff to reliably interpret, and quality control to mitigate high inter-reader variability.

WHO=World Health Organization. SpO2= peripheral capillary oxygen saturation. CRP=C-reactive protein. IL=interleukin.