Mycobacteriology (AFB) Culture (LAB877, LAB7000, LAB7001), gastric aspirate

Gastric aspirate specimens need to arrive in the lab for processing within 4 hours of collection.

Clinical Background
  • When there is clinical suspicion of pulmonary tuberculosis, the optimal specimen type for acid-fast bacteria (AFB) culture is sputum. When patients, particularly young children, are unable to produce quality sputum samples, gastric aspirate specimens is an alternative specimen type. Typically, this specimen is collected early in the morning after overnight fasting. Testing at least three specimens collected in three different days improves the sensitivity and predictive value.
  • Pulmonary tuberculosis is caused by Mycobacterium tuberculosis which requires an optimum pH of 6.2-7.3 for adequate growth. Because normal gastric pH is much lower at 1-2, neutralization of gastric acid specimens with a base has been shown to improve the yield of bacterial growth. Studies have shown that early neutralization, within 4 hours of collection of gastric aspirate specimens is optimal.
Common Pitfalls
  • Gastric acid specimens must arrive in the laboratory within 4 hours of collection. In order to assure proper processing, specimens should be received in the laboratory by 9:00 AM.
  • Specimens that arrive in the lab > 4 hours post-collection, or have delayed processing, will include a comment stating, “Specimen processed greater than 4 hours after collection.” The results of these cultures should be interpreted with caution.
Key Takeaways
  • Gastric aspirate specimens are acceptable when patients with suspicion of tuberculosis (TB) cannot produce an adequate sputum sample.
  • Gastric aspirate specimens require special consideration due to time-sensitive processing requirements.
  • Testing at least three specimens improves the sensitivity and predictive value.
References
  1. Abadco DL, Steiner P. Gastric lavage is better than bronchoalveolar lavage for isolation of Mycobacterium tuberculosis in childhood pulmonary tuberculosis. Pediatr Infect Dis J. 1992;11(9):735-738.
  2. Sharma R, Shukla S, Singh U, Gupta S, Jain A. Effect of time, temperature and pH on Mycobacterium tuberculosis culture positivity of gastric aspirate: An experimental study. Indian J Tuberc. 2023;70(2):190-196.

Written By: Lisa Wallace, DO and Carly Maucione

Reviewed By: Allison Eberly, PhD

Last Edited: 2024-09-08

Last Reviewed: 2024-09-08