Diagnosing Congenital Sucrase-Isomaltase Deficiency in Children: An Algorithm Using Combined Breath Testing

Abstract

Purpose

Disaccharidase deficiencies, including sucrase-isomaltase deficiency, can cause chronic gastrointestinal symptoms in children. While duodenal biopsies remain the diagnostic gold standard, results may be confounded by specimen handling variability or secondary mucosal injury. The noninvasive ¹³C sucrose breath test (¹³CSBT) accurately detects sucrase deficiency, including congenital sucrase-isomaltase deficiency (CSID), while the Trio-Smart^® breath test (BT) identifies small intestinal bacterial overgrowth (SIBO), a potential cause of secondary enzyme deficiency.

Methods

We conducted a retrospective review of 25 pediatric patients with disaccharidase deficiencies on duodenal biopsy and normal villous architecture. Patients underwent ¹³CSBT and/or Trio-Smart^® BT to evaluate for true CSID or SIBO. Clinical outcomes and treatment responses were assessed.

Results

Of 21 patients with low sucrase activities who completed ¹³CSBT, only 7 (33.3%) had abnormal results consistent with CSID. Six patients received sacrosidase, with three reporting symptom improvement. Of 15 patients who underwent Trio-Smart^® breath testing, 9 (60.0%) had abnormal results suggestive of SIBO and responded to antimicrobial treatment. Two patients had abnormal results on both tests. Interestingly, low palatinase levels were associated with abnormal ¹³CSBT in some cases, though not consistently.

Conclusion

Biopsy-based diagnosis may overestimate true CSID due to secondary causes or technical artifacts. Combined use of the ¹³CSBT and Trio-Smart^® BT provides a noninvasive strategy to help distinguish primary or secondary sucrase deficiency, improving diagnostic accuracy and avoiding unnecessary lifelong enzyme therapy. We propose a diagnostic algorithm that integrates biopsy and BT results to guide evaluation and reduce misclassification of CSID.

INTRODUCTION

Disaccharidase deficiencies, most commonly involving sucrase, lactase, maltase, and palatinase (isomaltase), are established causes of chronic gastrointestinal symptoms in children, including diarrhea, abdominal pain, and bloating. Enzyme activities are typically measured via duodenal biopsies obtained during esophagogastroduodenoscopy (EGD), which requires immediate freezing of specimens in dry ice to preserve enzymatic integrity [1,2]. Improper handling may result in falsely-low readings, and interpretation becomes more complex in the presence of mucosal injury or inflammation, which can lead to secondary enzyme deficiencies [3,4,5].

Congenital sucrase-isomaltase deficiency (CSID) is a rare genetic disorder caused by mutations that impair the structure or function of the sucrase-isomaltase (SI) complex [6,7]. Affected individuals are unable to digest sucrose and isomaltose, leading to persistent gastrointestinal symptoms unless treated with lifelong sacrosidase (Sucraid^®; QOL Medical) enzyme replacement therapy [6,8]. The ¹³C sucrose breath test (¹³CSBT) offers a noninvasive alternative to biopsy-based diagnosis and is available for both in-lab and at-home testing. This test measures exhaled ¹³CO following ingestion of a ¹³C-labeled sucrose substrate and has been shown to reliably identify CSID and monitor therapeutic response to sacrosidase [7,9,10].

When sucrase deficiency is accompanied by reductions in multiple disaccharidase enzymes on duodenal assay, the pattern may be referred to as pandisaccharidase deficiency [11,12]. This finding should prompt evaluation for underlying mucosal inflammatory conditions such as celiac disease, inflammatory bowel disease, allergic or eosinophilic gastroenteropathies, and small intestinal bacterial overgrowth (SIBO). In the absence of overt mucosal injury (e.g., villous atrophy), either CSID or adult-type hypolactasia—both relatively common genetic disorders—may explain the broader enzyme abnormalities. CSID has a prevalence of approximately 0.2–0.5% in the U.S. [7,13], while adult-type hypolactasia affects up to 65-70% of the global population, depending on ethnicity [14]. Because the SI complex also contributes to maltase and isomaltase activity, varying degrees of deficiency may occur, though normal maltase does not exclude CSID [15]. Thus, overlapping genetic causes can underlie seemingly widespread disaccharidase deficiency, and careful interpretation supported by clinical correlation, breath testing, and selective genetic analysis is essential.

While histopathology provides a definitive diagnosis for gut inflammatory diseases, it is not useful for conditions like SIBO, which lack consistent histologic findings and present with non-specific symptoms. As a result, SIBO remains a diagnostic challenge and often requires adjunctive testing [16,17]. The Trio-Smart^® breath test (BT) (Gemelli Biotech) is a novel home-based method that measures hydrogen (H₂), methane (CH₄), and hydrogen sulfide (H₂S) following ingestion of glucose or lactulose, enabling comprehensive detection of SIBO. While H₂ and CH₄ are well-established diagnostic markers, H₂S is a newer addition with evolving clinical significance [18,19]. According to North American Consensus, a rise in H₂ ≥20 ppm or CH₄ ≥10 ppm within 90 minutes of ingestion supports a diagnosis of SIBO [18].

This study evaluated the diagnostic utility of the ¹³CSBT and Trio-Smart^® BT in children with sucrase and other disaccharidase deficiencies identified on duodenal enzyme assay but normal villous architecture. Our aim was to distinguish primary CSID from secondary deficiencies due to SIBO or technical artifact, and to assess the clinical impact of breath testing on management, particularly the need for enzyme therapy.

MATERIALS AND METHODS

Study design

We conducted a retrospective review of 25 pediatric patients with negative celiac serologies who underwent EGD for evaluation of chronic gastrointestinal symptoms. Duodenal biopsies revealed sucrase and/or other disaccharidase deficiencies without villous atrophy. Following these findings, ¹³CSBT and Trio-Smart^® breath testing were offered to confirm sucrase deficiency or CSID, and to evaluate for SIBO, respectively. Of the 25 patients, 21 completed the ¹³CSBT, and 15 of these also underwent Trio-Smart^® breath testing. Test result outcomes and treatment responses were reviewed.

Data collection

Data were abstracted from electronic medical records using a standardized collection template. Variables included patient demographics (age at testing, gender, race, and ethnicity), relevant medical history, and reported gastrointestinal symptoms (abdominal pain, diarrhea, constipation, vomiting, weight loss or failure to thrive, nausea, and/or bloating). Diagnostic data included results of the ¹³CSBT, Trio-Smart^® BT (H ₂, CH₄, and H₂S levels), and duodenal disaccharidase activity (lactase, sucrase, maltase, and palatinase). Duodenal biopsy findings and any associated histologic abnormalities, particularly with villous atrophy, were recorded. Additional variables included the clinical indication for testing, sacrosidase (Sucraid^®) use, follow-up, and treatment response. Treatment response was defined based on physician documentation of symptom improvement, persistence, or resolution following therapy. For patients treated with antibiotics for SIBO, response was assessed within 2–6 weeks of therapy using clinic notes or parent-reported symptom change. For sacrosidase (Sucraid^®), improvement was defined as reduction in abdominal pain, diarrhea, or bloating during routine follow-up, typically 4–12 weeks after initiation. Duration of follow-up was variable (range 1–12 months), and patients without follow-up documentation were classified as having indeterminate response.

Diagnostic testing

All patients underwent duodenal disaccharidase enzyme analysis performed at our institutional clinical laboratory. Reference ranges were: sucrase ≥25 µmoL/min/g protein, lactase ≥15 µmoL/min/g protein, maltase ≥100 µmoL/min/g protein, and palatinase ≥5 µmoL/min/g protein. Enzyme activities below these thresholds were considered abnormal.

The ¹³CSBT was used to evaluate for CSID. After ingestion of a standardized dose of ¹³C-labeled sucrose, breath samples were collected at baseline and at regular intervals up to 90 minutes. Abnormal results were defined as cumulative ¹³CO₂ recovery <5.10% in females and <3.91% in males, consistent with established pediatric reference values.

The Trio-Smart^® BT was used to assess for SIBO. Patients ingested a glucose or lactulose substrate, and breath samples were collected every 15–20 minutes over 2–3 hours. Abnormal results were defined according to North American Consensus: a rise in H₂ ≥20 ppm within 90 minutes or CH₄ ≥10 ppm at any point. H₂S values were also recorded, with ≥3 ppm considered abnormal for exploratory purposes.

The interval between completion of the duodenal disaccharidase enzyme analysis, ¹³CSBT, and Trio-Smart^® BT was recorded for each patient to assess the potential impact of timing on diagnostic concordance.

Statistical analysis

Descriptive statistics summarized patient demographics, clinical features, disaccharidase assay results, and BT outcomes. Categorical data were reported as counts and percentages, while continuous data were presented as ranges or medians where appropriate. No inferential statistical analyses were conducted due to the small sample size and exploratory nature of the study.

Ethics statement

This study was approved by the Institutional Review Board (IRB #00360702) and conducted in accordance with institutional ethical standards. Due to its retrospective design, informed consent was waived.

RESULTS

Overview

A total of 25 pediatric patients (14 males and 11 females; median age 13 years [range 4–17]) underwent evaluation for CSID using a combination of duodenal disaccharidase analysis, ¹³CSBT, and/or the Trio-Smart^® BT. Among the 25 patients, 16 were White (64.0%), 6 were Black (24.0%), 2 were Hispanic (8.0%), and 1 was Asian (4.0%). Common presenting symptoms included a combination of abdominal pain (72%), diarrhea (52%), weight loss (24%), vomiting (20%), constipation (16%), nausea (12%), and bloating (4%). A detailed summary of patient demographics, presenting symptoms, and test findings is provided in Table 1. The average intervals were 32 days (median 14, range 0–158) between duodenal disaccharidase analysis and the ¹³CSBT, 83 days (median 48, range 20–210) between disaccharidase analysis and the Trio-Smart^® BT, and 44 days (median 28, range 6–131) between the two breath tests. Genetic confirmation of CSID was not performed in any patients.

Table 1. Patient characteristics and results of breath testing and disaccharidase analysis.

Patient	Age (yr)	Sex	Ethnicity	Medical history	Symptoms	13CSBT results	13CSBT value (%)	Trio-Smart BT results	H2 (ppm)	CH4 (ppm)	H2S (ppm)	Lactase	Sucrase	Maltase	Palatinase	Duodenal biopsy findings
1	15	Female	White	Constipation	Blood in stool	Abnormal	1.76 (L)	Abnormal (SIBO)	23.52 (H)	0.39	1.11	11.6 (L)	23.7 (L)	93.4 (L)	9.1	Normal
2	17	Male	Black	None	Abdominal pain, nausea, weight loss	Abnormal	3.3 (L)	Abnormal (Methane)	10	10.99 (H)	0.9	1 (L)	7.5 (L)	30.5 (L)	2.3 (L)	Normal
3	14	Female	White	Irritable bowel syndrome, type 1 diabetes, polycystic ovarian syndrome, vocal cord dysfunction	Abdominal pain, reflux, diarrhea	Abnormal	1.88 (L)	Normal	17.85	1.32	1.6	9 (L)	14.4 (L)	54 (L)	5.6	Normal
4	17	Male	White	Ruptured tympanic membrane	Diarrhea	Abnormal	3.23 (L)	Normal	15.62	0.5	1.53	2 (L)	8.7 (L)	37.4 (L)	3.7 (L)	Normal
5	4	Male	Black	Alagille syndrome post liver transplantation	Abdominal pain, diarrhea	Abnormal	3.43 (L)	Not available	-	-	-	2.3 (L)	10.2 (L)	73.3 (L)	5.0	Normal
6	9	Female	Black	Sinding-Larsen-Johansson syndrome, juvenile idiopathic arthritis	Abdominal pain, nausea, weight loss	Abnormal	4.97 (L)	Not available	-	-	-	7.7 (L)	24.4 (L)	88.7 (L)	8.7	Normal
7	15	Female	White	Crohn's disease	Abdominal pain, weight loss, hematochezia	Abnormal	1.96 (L)	Not available	-	-	-	3.7 (L)	12.7 (L)	39.6 (L)	4 (L)	Normal
8	5	Male	White	Constipation, failure to thrive	Abdominal pain, bloating	Normal	7.98	Abnormal (SIBO)	66.74 (H)	3.24	1.17	5.7 (L)	17.5 (L)	65.8 (L)	7.8	Normal
9	9	Female	Black	Constipation	Abdominal pain, constipation	Normal	5.86	Abnormal (SIBO)	42.31 (H)	0.59	1.56	5.5 (L)	20 (L)	66.1 (L)	6.2	Normal
10	11	Male	Asian	None	Abdominal pain, diarrhea	Normal	8.29	Abnormal (SIBO)	27.5 (H)	0.14	0.76	2.6 (L)	22.5 (L)	71.2 (L)	7.6	Normal
11	15	Male	White	Crohn's disease	Diarrhea	Normal	8.18	Abnormal (SIBO)	37.63 (H)	2.28	1	10.4 (L)	23 (L)	72.3 (L)	6.7	Normal
12	16	Male	White	Eosinophilic esophagitis	Abdominal pain, constipation	Normal	7.96	Abnormal (SIBO)	51.47 (H)	0	1.62	5 (L)	17.8 (L)	59.4 (L)	5.5	Normal
13	16	Female	White	Depression, anxiety, constipation, left uteropelvic junction obstruction	Abdominal pain, nausea	Normal	7.47	Abnormal (SIBO)	25.38 (H)	4.02	3.35 (H)	14.4 (L)	23.8 (L)	83.5 (L)	5.6	Normal
14	11	Male	Black	None	Abdominal pain, constipation, diarrhea	Normal	7.42	Normal	14.64	3.49	1.2	9 (L)	17.9 (L)	76.6 (L)	7.4	Focal acute duodenitis
15	12	Male	White	Constipation, cecostomy tube dependence, SIBO	Constipation	Normal	5.38	Normal	13.03	0	2.41	9.7 (L)	24.5 (L)	92.8 (L)	9.5	Normal
16	13	Female	White	Ehlers-Danlos syndrome, gastroesophageal reflux disease, constipation	Abdominal pain, vomiting, early satiety, weight loss	Normal	6.94	Normal	12.35	0	1.85	5.7 (L)	15.3 (L)	50.8 (L)	5.1	Normal
17	14	Female	Hispanic	Functional abdominal pain	Abdominal pain	Normal	8.75	Normal	4.73	2.45	0.92	1.3 (L)	24.2 (L)	87.6 (L)	4.7 (L)	Normal
18	12	Male	Hispanic	Chronic abdominal pain	Abdominal pain, vomiting, diarrhea	Normal	4.3	Not available	-	-	-	2.4 (L)	22.4 (L)	78.3 (L)	7.4	Normal
19	12	Male	White	Cyclic vomiting syndrome, eosinophilic esophagitis, constipation, migraines	Abdominal pain, vomiting, diarrhea	Normal	9.03	Not available	-	-	-	6.2 (L)	10.6 (L)	35.5 (L)	5.3	Normal
20	13	Female	White	IgA deficiency	Abdominal pain, diarrhea	Normal	9.39	Not available	-	-	-	11.1 (L)	13.2 (L)	44.7 (L)	5.6	Normal
21	15	Male	White	None	Abdominal pain, diarrhea	Normal	8.07	Not available	-	-	-	10.7 (L)	16 (L)	71.7 (L)	5.4	Normal
22	12	Female	White	Crohn's disease	Abdominal pain, diarrhea, weight loss	Deferred - positive SIBO testing	-	Abnormal (SIBO)	71.9 (H)	6.36	1.28	10.4 (L)	23.3 (L)	96.9 (L)	7.5	Gastric mucin cell metaplasia
23	7	Male	White	Double-outlet right ventricle, Dandy-Walker syndrome, hypotonia, developmental delays	Vomiting, reflux	Deferred - normal repeat enzyme analysis	-	Not available	-	-	-	12.5 (L)	22 (L)	64 (L)	5.9	Normal
24	10	Female	Black	Chromosomal deletion, global delay, asthma, selective IgA deficiency, lymphocytic colitis	Diarrhea, mucus in stool, weight loss	Deferred - normal repeat enzyme analysis	-	Not available	-	-	-	7.1 (L)	16.2 (L)	83.7 (L)	2.2 (L)	Normal
25	16	Male	White	Trisomy 21, presumed ulcerative colitis	Vomiting, diarrhea, mucus in stool	Deferred - diagnosed with small bowel Crohn’s disease	-	Not available	-	-	-	3.2 (L)	8.1 (L)	30.9 (L)	3.9 (L)	Normal

Disaccharidase enzymes (lactase, sucrase, maltase, and palatinase) are measured in µmoL/min/g protein. Reference ranges: disaccharidase activity (µmoL/min/g protein): lactase: 15–45.5; sucrase: 25–69.9; maltase: 100–224.4; palatinase: 5–26.3. ¹³CSBT (% cumulative dose recovered): male: >3.91; female: >5.10. Trio-Smart BT (ppm, peak gas levels): H₂: <28.16; CH₄: <10 ppm; H₂S: <3.

¹³CSBT: ¹³C sucrose breath test, BT: breath test, CH₄: methane, H: high, H₂: hydrogen, H₂S: hydrogen sulfide, IgA: immunoglobulin A, L: low, SIBO: small intestinal bacterial overgrowth.

Disaccharidase analysis and biopsy findings

All 25 patients underwent duodenal disaccharidase testing. Sucrase deficiency was identified in all 25 patients (100.0%), with a median sucrase value of 17.8 µmoL/min/g protein (range: 7.5–24.5). Concomitant lactase deficiency (lactase <15 µmoL/min/g) and maltase deficiency (maltase <100 µmoL/min/g protein) were also observed in all 25 patients (100.0%). Palatinase deficiencies were less common, present in 24% of patients.

Duodenal biopsy was performed in all patients and was normal in 23/25 (92.0%). Two patients had abnormal histologic findings: one with focal acute duodenitis and one with gastric mucin cell metaplasia.

¹³CSBT

The ¹³CSBT was completed in 21/25 patients (84.0%), with 7 (33.3%) showing abnormal results. The median ¹³CO₂ result among abnormal cases was 3.23% (range: 1.76–4.97%), while normal results ranged from 4.3% to 9.39%. Patients with abnormal ¹³CSBT results all had low sucrase levels and variable symptoms. Six patients with abnormal ¹³CSBT and either normal or unavailable Trio-Smart^® breath testing were prescribed sacrosidase (Sucraid^®). In four patients, the ¹³CSBT was not completed due to alternative explanations for disaccharidase deficiency: two had normalization of enzyme levels on repeat biopsy, one was ultimately diagnosed with Crohn’s disease (initially presumed to be ulcerative colitis), and one had testing deferred in the setting of confirmed SIBO on the Trio-Smart^® BT.

Trio-Smart^® BT

The Trio-Smart^® BT was completed in 15/25 patients (60.0%), with 9 (60.0%) showing abnormal results consistent with SIBO or methane-positive overgrowth. H₂S values ranged from 0.14 to 6.36 ppm, with two patients exceeding the exploratory threshold of 3 ppm. All patients with abnormal Trio-Smart^® BT results had sucrase activities below the normal reference range, suggesting that SIBO may contribute to secondary sucrase deficiency. Six patients had normal ¹³CSBT but abnormal Trio-Smart^® BT results, supporting a secondary etiology for their disaccharidase deficiency.

Diagnostic overlap and interpretation

Among the 15 patients who completed both ¹³CSBT and Trio-Smart^® testing, diagnostic overlap was observed (Table 2). Within this subgroup:

Table 2. Diagnostic interpretation of sucrase deficiency using combined breath testing.

13C sucrose breath test	Trio-Smart breath test			Total
	Abnormal	Normal	Not available
Abnormal	2 (8.0)	2 (8.0)	3 (12.0)	7 (28.0)
	Mixed pathology	Consistent with CSID
Normal	6 (24.0)	4 (16.0)	4 (16.0)	14 (56.0)
	Suggestive of SIBO	Likely false-positive disaccharidase assay
Test deferred	1 (4.0)	-	3 (12.0)	4 (16.0)
	Deferred due to positive SIBO		Deferred due to alternative explanations*
Total	9 (36.0)	6 (24.0)	10 (40.0)	25 (100.0)

Values are presented as number (%).

Percentages are calculated out of the full cohort of 25 patients. Subgroup percentages for the 15 patients who completed both tests are described in text.

CSID: congenital sucrase-isomaltase deficiency, SIBO: small intestinal bacterial overgrowth.

^*Two patients had normal enzyme activity on repeat duodenal disaccharidase testing, and one was reclassified as having small bowel Crohn’s disease rather than ulcerative colitis.

Six patients (40.0%) had normal ¹³CSBT but abnormal Trio-Smart^® BT, suggestive of secondary sucrase deficiency due to SIBO.

Four patients (26.7%) had both tests normal, potentially indicating resolving enzyme deficiency or false-positive disaccharidase results.

Two patients (13.3%) had abnormal ¹³CSBT but normal Trio-Smart^® BT, consistent with primary CSID.

Two patients (13.3%) had both tests abnormal, where mixed pathology (CSID+SIBO) could not be excluded.

These patterns reflect the diagnostic complexity in interpreting isolated disaccharidase deficiencies and the role of noninvasive breath testing in differentiating primary from secondary etiologies.

Follow-up and treatment outcomes

Sacrosidase (Sucraid^®) therapy was initiated in 6 patients (24.0%) with abnormal ¹³CSBT results. Clinical improvement was defined as physician- or parent-reported reduction in gastrointestinal symptoms documented at follow-up. Of those, three had documented clinical improvement, one discontinued due to lack of efficacy, and follow-up was incomplete for the remaining two. Repeat breath testing was performed in four patients, with normalization documented in two (suggesting resolution of secondary causes). Antibiotics for SIBO were effective in most with abnormal Trio-Smart^® results.

Several patients with abnormal Trio-Smart^® BT results had incomplete follow-up or transitioned to adult care, limiting assessment of long-term outcomes. One patient with abnormal results on both breath tests remained positive on repeat ¹³CSBT testing, reinforcing the need for ongoing monitoring in patients with possible CSID and overlapping SIBO.

DISCUSSION

This study highlights the diagnostic complexity of sucrase deficiency in children and supports incorporating noninvasive breath testing into the diagnostic approach. While duodenal enzyme assays remain the gold standard, their accuracy is highly dependent on proper specimen handling and can be confounded by secondary mucosal injury, leading to false-positive results [20,21]. Adjunctive strategies like breath testing may improve diagnostic accuracy.

Previous studies have also reported the limitations of biopsy-based disaccharidase analysis, noting that falsely-low enzyme activities may result from technical factors such as inadequate freezing, or from secondary causes including mucosal inflammation, celiac disease, or intestinal infections [7,20]. The ¹³CSBT offers a validated, noninvasive alternative that correlates with known SI gene mutations and response to sacrosidase (Sucraid^®), as seen in the studies by Robayo-Torres et al. [9] and Dale et al. [10]. Similarly, SIBO is an increasingly recognized contributor to secondary sucrase deficiency [17,22]. The application of comprehensive breath testing—including H₂, CH₄, and H₂S analysis—has emerging diagnostic utility in this specific context [16].

Our findings reveal that a substantial proportion of patients with biopsy-detected sucrase deficiency had normal ¹³CSBT results, suggesting that biopsy-based assays alone may overestimate the prevalence of CSID. Only 7 of 21 tested patients (33.3%) had abnormal ¹³CSBT results, suggestive of CSID. Among those, six were started on sacrosidase (Sucraid^®), and three experienced symptomatic improvement. Conversely, the majority of patients with low sucrase activity on biopsy had normal ¹³CSBT results, supporting the role of secondary factors or technical artifact.

SIBO emerged as a common secondary contributor to apparent sucrase deficiency. Just over half (53%) of patients undergoing Trio-Smart^® breath testing met criteria for SIBO, and one-fourth had normal ¹³CSBT but abnormal SIBO results. This diagnostic pattern supports the theory that SIBO-associated mucosal injury may transiently suppress brush border enzyme activity, including sucrase, without an underlying genetic defect. In such cases, treatment of SIBO alone led to symptom improvement, obviating the need for enzyme replacement therapy. This demonstrates the value of comprehensive testing to avoid misdiagnosis and unnecessary lifelong treatment.

An interesting observation was the association between biopsy-detected low palatinase activity and abnormal ¹³CSBT results in a subset of patients, suggesting that palatinase may serve as a surrogate marker for CSID. However, its variability among patients with normal BT results limits its reliability. Further studies are warranted to determine whether low palatinase activity enhances diagnostic specificity or simply reflect subclinical mucosal disruption. Similarly, the sucrase-lactase ratio (S:L) may offer additional diagnostic value. A ratio <1, particularly in the setting of preserved lactase activity, has been proposed as a supportive marker for CSID and may help distinguish primary enzyme deficiencies from secondary or technical causes [23].

The combined application of ¹³CSBT and Trio-Smart^® breath testing enabled stratification of patients into probable CSID, secondary enzyme suppression (e.g., from SIBO), or false-positive biopsy results. Two patients had abnormalities on both tests, raising the possibility of dual pathology, such as coexisting CSID and SIBO. This diagnostic overlap suggests a role for repeat testing post-treatment and stresses the dynamic nature of small bowel mucosal function.

Taken together, these findings support a revised diagnostic paradigm in which abnormal disaccharidase activity on biopsy prompts reflexive, noninvasive breath testing to confirm true deficiency and guide management. This strategy offers several advantages: it reduces dependence on invasive or technically sensitive biopsy procedures, minimizes diagnostic error, guides appropriate therapy, and prevents unnecessary initiation of long-term sacrosidase (Sucraid^®) replacement therapy. Fig. 1 illustrates a diagnostic algorithm to sucrase deficiency—with and without accompanying disaccharidase deficiencies—integrating relevant clinical investigations. From a practical standpoint, the concurrent use of sucrose as a substrate for both the ¹³CSBT and Trio-Smart^® BT is an intriguing and potentially cost-efficient strategy. However, the accuracy and diagnostic reliability of using sucrose as a substrate for each BT would require further validation in prospective studies before it can be broadly implemented.

Fig. 1. Diagnostic algorithm for sucrase deficiency: distinguishing CSID from secondary causes. ^*Sucrase, isomaltase, and maltase come from the same SI enzyme complex in the brush border of the small intestine. Due to the common precursor, SI gene mutations can impair all three enzyme activities to varying degrees. ^†Hydrogen breath tests using lactulose or other sugars (glucose or lactose) are used to assess for SIBO. Newer methods also measure CH₄ and H₂S gases. ^‡SI gene testing and/or repeat biopsy-based disaccharidase analysis can be valuable in evaluating complex or inconclusive cases.

BT: breath test, CH₄: methane, CSID: congenital sucrase-isomaltase deficiency, H₂: hydrogen, H₂S: hydrogen sulfide, S:L: sucrase-lactase ratio, SI: sucrase-isomaltase, SIBO: small intestinal bacterial overgrowth.

The main limitations of this study are its retrospective, single-center design and modest sample size. Genetic confirmation of CSID was not performed in any patients, which would have strengthened diagnostic certainty in cases of abnormal ¹³CSBT. Of the 25 patients, only 21 underwent the ¹³CSBT and 15 underwent the Trio-Smart^® BT, resulting in incomplete overlap across modalities and introducing uncertainty when extrapolating findings. The absence of a healthy control group prevents estimation of specificity and limits our ability to quantify the discriminative performance of the proposed diagnostic algorithm. Follow-up data were incomplete for some patients, and both breath testing protocols and dietary intake varied, potentially introducing procedural variability. Finally, although complementary use of the ¹³CSBT and Trio-Smart^® BT may offset limitations of biopsy-based enzyme assays, both tests carry inherent sources of measurement error—particularly in younger children, where feasibility and test performance may differ.

Future directions should include prospective validation of this combined BT strategy, evaluation of optimal testing thresholds in pediatric populations, and integration of SI gene sequencing to enhance diagnostic precision. Development of pediatric-specific guidelines for the evaluation and management of disaccharidase deficiencies would also be highly valuable to clinicians.

In conclusion, combined ¹³C sucrose and Trio-Smart^® breath testing may improve diagnostic accuracy in suspected CSID by distinguishing primary from secondary sucrase deficiency due to SIBO. While duodenal enzyme assays remain an important diagnostic tool, adjunctive breath testing may reduce misdiagnosis and unnecessary enzyme therapy, supporting a more personalized, efficient approach to care.