Paper I
Question
Lab diagnosis of helicobacter infection
Answer
Laboratory diagnosis of Helicobacter pylori infection uses both invasive (endoscopy-based) and non-invasive methods, chosen according to clinical context (whether endoscopy is already indicated) and purpose (initial diagnosis versus confirming eradication).
Invasive (requiring endoscopic biopsy):
- Rapid urease test (CLO test) — the biopsy sample is placed in a urea-containing medium with a pH indicator; the organism’s abundant urease hydrolyses urea to ammonia, raising pH and producing a colour change — rapid, low-cost, widely used as a first-line test at endoscopy.
- Histopathology — biopsy sections (often with Giemsa or special silver stains) show curved/spiral organisms in the gastric mucus layer, along with associated chronic gastritis; allows assessment of mucosal damage/atrophy/malignancy alongside organism detection.
- Culture — technically demanding (requires microaerophilic conditions, prolonged incubation, selective media); not routinely used for primary diagnosis but valuable for antibiotic susceptibility testing in treatment-refractory cases.
- Biopsy-based PCR/molecular testing — detects the organism and can identify antibiotic-resistance mutations (e.g., clarithromycin resistance).
Non-invasive:
- Urea breath test — the patient ingests urea labelled with a non-radioactive (¹³C) or radioactive (¹⁴C) isotope; bacterial urease liberates labelled CO2, detected in exhaled breath — high sensitivity/specificity, and the test of choice for confirming eradication after treatment (must be done ≥4 weeks after completing therapy, with prior discontinuation of proton-pump inhibitors, to avoid false-negative results).
- Stool antigen test — detects H. pylori antigen in stool using an enzyme immunoassay; useful for both initial diagnosis and confirming eradication.
- Serology — detects IgG antibody against H. pylori; useful for epidemiological screening, but cannot distinguish active infection from past exposure (antibody persists after eradication), so it is not suitable for confirming successful treatment.
Practical note: proton-pump inhibitors and antibiotics/bismuth should be withheld for an appropriate period before urease-based tests (rapid urease test, urea breath test) and culture, as they can suppress the organism and cause false-negative results.