What lab tests are used to diagnose sugar intolerance?

Laboratory Tests for Sugar Intolerance

The primary laboratory tests for diagnosing sugar intolerance are hydrogen breath tests using specific sugar substrates (lactose, fructose, sucrose), which measure breath hydrogen concentration ≥20 ppm above baseline to detect carbohydrate malabsorption. 1, 2

Important Distinction: Sugar Intolerance vs. Diabetes

The term "sugar intolerance" typically refers to carbohydrate malabsorption syndromes (lactose, fructose, sucrose intolerance), which are completely different from glucose intolerance/diabetes. If you are asking about diabetes screening, see the diabetes testing section below. If you are asking about digestive symptoms from specific sugars, continue reading this section.

Tests for Carbohydrate Malabsorption (True "Sugar Intolerance")

Hydrogen Breath Tests

• Lactose hydrogen breath test uses 20g lactose load and measures breath hydrogen every 30 minutes for 3 hours 1, 2
• Fructose hydrogen breath test uses 20g fructose load with similar protocol 1, 3
• Sucrose hydrogen breath test can use either hydrogen-methane or 13C-sucrose breath test methodology 4
• Triple sugar screen test (TSST) combines all three sugars in one test, though individual tests may be more specific 1

Interpretation Criteria

• Positive test: Breath hydrogen concentration ≥20 ppm above baseline indicates sugar malabsorption 1, 2
• Timing matters: Symptoms occurring during the 3-hour testing period suggest true intolerance (65% of malabsorbers develop symptoms during testing) 3
• Methane measurement: Some patients produce methane instead of hydrogen, so dual hydrogen-methane testing improves sensitivity 4

Test Performance Characteristics

• Fructose breath test repeatability: Moderate for intolerance status (87% absolute agreement, kappa 0.72) but limited for malabsorption status (53% absolute agreement) 5
• Sucrose malabsorption incidence: 26.5-40% in adults with functional GI symptoms, depending on test methodology 4
• Clinical correlation: Approximately 16% of children with functional abdominal symptoms have sugar intolerance on TSST 1

Gold Standard Confirmation

• Duodenal biopsy with enzyme assay (particularly sucrase enzyme assay) is the gold standard for diagnosing enzyme deficiencies, but breath tests serve as effective non-invasive screening tools 4

Tests for Glucose Intolerance/Diabetes (If This Is Your Question)

Primary Diagnostic Tests

• Fasting plasma glucose (FPG) ≥126 mg/dL (7.0 mmol/L) after 8-hour fast - preferred screening test 6
• Hemoglobin A1C ≥6.5% (48 mmol/mol) using NGSP-certified laboratory method 6, 7
• 2-hour plasma glucose ≥200 mg/dL (11.1 mmol/L) during 75g oral glucose tolerance test 6, 7
• Random plasma glucose ≥200 mg/dL with classic symptoms (polyuria, polydipsia, weight loss) 6

Prediabetes Criteria

• A1C 5.7-6.4% 6
• Fasting glucose 100-125 mg/dL (impaired fasting glucose) 6
• 2-hour OGTT 140-199 mg/dL (impaired glucose tolerance) 6

Confirmation Requirements

• Two abnormal test results required in absence of unequivocal hyperglycemia - can be same test repeated or two different tests (e.g., A1C and FPG) 6, 7, 8

When NOT to Use A1C

• Use only plasma glucose criteria in: pregnancy (second/third trimester), sickle cell disease, G6PD deficiency, HIV, hemodialysis, recent blood loss/transfusion, erythropoietin therapy, hemoglobin variants 6, 8

Clinical Algorithm

For digestive symptoms (bloating, gas, diarrhea after eating):

1. Start with hydrogen-methane breath tests for specific sugars based on dietary triggers 2, 4
2. If positive and symptoms correlate, trial elimination diet of offending sugar 3
3. Consider duodenal biopsy if breath tests inconclusive but high clinical suspicion 4

For screening diabetes/glucose intolerance:

1. Begin with FPG (most practical) or A1C in patients with risk factors 6, 7, 8
2. Confirm any positive result with repeat testing 6, 7
3. Use OGTT if FPG normal but high clinical suspicion 6

Critical Pitfalls to Avoid

• Don't confuse carbohydrate malabsorption with diabetes - these require completely different testing approaches [1,2 vs 6]
• Breath test preparation: Patients must consume adequate carbohydrates (≥150g) for 3 days before OGTT to avoid false positives 6
• Sample handling for glucose tests: Blood must be processed promptly in tubes with glycolytic inhibitors to prevent falsely low values 6, 8
• Breath test repeatability limitations: Malabsorption status has poor repeatability (kappa 0.05), so clinical correlation is essential 5