12.4 Gastrointestinal Function & Diagnostic Testing
Key Takeaways
- pH-multichannel intraluminal impedance is the test that quantifies both acid and non-acid reflux and links episodes to symptoms; an upper GI series evaluates anatomy, not reflux
- A hydrogen breath test is positive for carbohydrate malabsorption at a rise of about 20 ppm above baseline, and for bacterial overgrowth when that rise occurs within roughly 90 minutes
- Fecal elastase-1 below 200 µg/g indicates exocrine pancreatic insufficiency and is not affected by enzyme replacement taken during the test
- Stool alpha-1-antitrypsin identifies protein-losing enteropathy, and a stool pH below 5.5 with reducing substances points to carbohydrate malabsorption
- Plasma citrulline reflects functional enterocyte mass and helps predict parenteral nutrition dependence in short bowel syndrome
Matching the Test to the Nutrition Question
Gastrointestinal testing enters pediatric nutrition practice at three decision points: why is this child not tolerating feeds, why is this child not absorbing what is delivered, and is the route of feeding safe and appropriate. Choosing the wrong study wastes time and, in a child with faltering growth, wastes weeks.
| Nutrition question | Test that answers it |
|---|---|
| Is there a structural obstruction, malrotation, or stricture? | Upper GI series with small bowel follow-through |
| Is reflux frequent enough - and linked to symptoms - to justify therapy or post-pyloric feeding? | pH-multichannel intraluminal impedance (pH-MII) |
| Is delayed gastric emptying causing vomiting and intolerance? | Gastric emptying scintigraphy |
| Is there mucosal disease (eosinophilic esophagitis, celiac, inflammatory bowel disease)? | Endoscopy with biopsy |
| Is fat being malabsorbed, and is the pancreas the cause? | 72-hour fecal fat / coefficient of fat absorption, fecal elastase-1 |
| Is carbohydrate being malabsorbed, or is there bacterial overgrowth? | Hydrogen/methane breath testing, stool pH and reducing substances |
| Is protein being lost through the gut? | Stool alpha-1-antitrypsin |
| How much functional small bowel remains? | Plasma citrulline, imaging-based remnant length |
Swallowing safety is a separate question answered by videofluoroscopic swallow study or fiberoptic endoscopic evaluation of swallowing, covered with dysphagia management rather than here.
Imaging, Reflux, and Motility Studies
Upper GI series. Contrast fluoroscopy defines anatomy: the position of the ligament of Treitz (malrotation), duodenal webs, strictures, tracheoesophageal fistula, and post-surgical anatomy. It is emphatically not a reflux test - transient reflux during a contrast study is common in normal infants and its absence proves nothing.
pH-multichannel intraluminal impedance. Impedance detects any bolus movement, acid or not, while the pH sensor characterizes it. This matters in pediatrics because much infant and tube-fed reflux is non-acid - especially in children on acid suppression or receiving continuous feeds - and would be invisible to pH monitoring alone. The output that changes management is the symptom association: retching, desaturation, or apnea that reliably coincides with reflux episodes supports post-pyloric feeding or fundoplication; symptoms with no temporal relationship argue against blaming reflux at all.
Gastric emptying scintigraphy. A radiolabeled meal is imaged over time; in older children a 4-hour protocol is standard, and pediatric meal composition varies by center, so compare to that laboratory's norms. Delayed emptying supports smaller volumes, continuous or post-pyloric delivery, lower-fat and lower-fiber formula, and prokinetic consideration. Rapid emptying after fundoplication or pyloroplasty points toward dumping instead - a different prescription entirely.
Manometry (antroduodenal, colonic, anorectal) is reserved for suspected motility disorders such as pediatric intestinal pseudo-obstruction and Hirschsprung disease. MR enterography assesses Crohn disease activity and remnant bowel without ionizing radiation.
Endoscopy, Breath Tests, and Stool Studies
Endoscopy with biopsy is definitive for mucosal disease. Nutrition-relevant thresholds: at least 15 eosinophils per high-power field on esophageal biopsy for eosinophilic esophagitis; villous atrophy with crypt hyperplasia (graded by the Marsh classification) for celiac disease; and characteristic inflammation for inflammatory bowel disease. The specialist's practical role is timing - celiac serology and biopsy must be performed while the child is still eating gluten, so a dietitian who starts a gluten-free diet before testing has destroyed the diagnosis.
Hydrogen and methane breath testing. After a substrate load, unabsorbed carbohydrate fermented by bacteria produces hydrogen (or methane) that is exhaled and measured.
- Lactose or fructose malabsorption: a rise of about 20 ppm above baseline over the test period is the conventional positive threshold.
- Small intestinal bacterial overgrowth (SIBO): an early rise - roughly 20 ppm within about 90 minutes of a glucose or lactulose load - suggests fermentation proximal to the colon. SIBO is common after loss of the ileocecal valve and in dilated, dysmotile short bowel, and it presents as feeding intolerance, gas, D-lactic acidosis, and stalled advancement.
- Roughly 15-20% of people are hydrogen non-producers, which is why methane is co-measured.
Stool studies are the workhorse of malabsorption assessment.
| Test | Threshold / finding | Meaning |
|---|---|---|
| 72-hour quantitative fecal fat with a recorded fat intake | Coefficient of fat absorption (CFA) = (fat in − fat out) / fat in × 100; roughly ≥93% is normal in children over 6 months (lower in young infants) | Gold standard for steatorrhea; distinguishes malabsorption from inadequate intake |
| Fecal elastase-1 | <200 µg/g | Exocrine pancreatic insufficiency; unaffected by enzyme replacement, so PERT need not be held. Watery stool can dilute the sample and cause false lows |
| Stool alpha-1-antitrypsin | Elevated | Protein-losing enteropathy - Fontan physiology, intestinal lymphangiectasia, severe inflammation |
| Stool pH and reducing substances | pH <5.5 with positive reducing substances | Carbohydrate malabsorption with colonic fermentation |
| Stool electrolytes and osmotic gap | Large gap = osmotic; small gap = secretory | Separates diarrhea that stops with fasting from diarrhea that does not |
| Fecal calprotectin | Elevated | Mucosal inflammation; helps separate inflammatory bowel disease from functional abdominal pain |
Plasma citrulline deserves its own mention: this non-protein amino acid is produced almost exclusively by small bowel enterocytes, so a low value (commonly cited around <15-20 µmol/L) reflects reduced functional enterocyte mass and correlates with continued parenteral nutrition dependence in short bowel syndrome. Interpret it with caution in renal impairment, which raises levels.
Finally, remember that an abdominal radiograph remains the fastest nutrition-relevant study in a deteriorating infant: pneumatosis intestinalis or portal venous gas defines Bell stage II necrotizing enterocolitis and stops enteral feeding immediately.
A tube-fed 2-year-old on continuous gastric feeds and a proton pump inhibitor has daily retching and intermittent desaturations. The team wants to know whether reflux explains the episodes. Which study is most appropriate?
A child with short bowel syndrome and no ileocecal valve has recurring bloating, gas, and stalled enteral advancement. A glucose hydrogen breath test shows a 25 ppm rise above baseline at 60 minutes. What does this indicate?
Which statement about fecal elastase-1 testing is correct?