7
Testing for TB Disease
➤ The panel recommends that acid-fast bacilli (AFB) smear
microscopy be performed, rather than no AFB smear microscopy,
in all patients suspected of having pulmonary TB (S-M).
Remarks: False-negative results are sufficiently common that a negative AFB smear
result does not exclude pulmonary TB. Similarly, false-positive results are sufficiently
common that a positive AFB smear result does not confirm pulmonary TB. Testing of
3 specimens is considered the normative practice in the United States and is strongly
recommended by the Centers for Disease Control and Prevention and the National
Tuberculosis Controllers Association in order to improve sensitivity given the pervasive
issue of poor sample quality. Providers should request a sputum volume of ≥3 mL, but
the optimal volume is 5–10 mL. Concentrated respiratory specimens and fluorescence
microscopy are preferred.
➤ The panel suggests that both liquid and solid mycobacterial
cultures be performed, rather than either culture method alone,
for every specimen obtained from an individual with suspected
TB disease (C-L).
Remarks: The conditional qualifier applies to performance of both liquid and solid
culture methods on all specimens. At least liquid culture should be done on all specimens
since culture is the gold standard microbiologic test for the diagnosis of TB disease.
The isolate recovered should be identified according to the Clinical and Laboratory
Standards Institute guidelines and the American Society for Microbiolog y Manual of
Clinical Microbiolog y.
➤ The panel suggests performing a diagnostic nucleic acid
amplification test (NAAT), rather than not performing a NAAT, on
the initial respiratory specimen from patients suspected of having
pulmonary TB (C-L).
Remarks: In AFB smear-positive patients, a negative NAAT makes TB disease unlikely.
In AFB smear-negative patients with an intermediate to high level of suspicion for
disease, a positive NAAT can be used as presumptive evidence of TB disease, but a
negative NAAT cannot be used to exclude pulmonary TB. Appropriate NAAT include
the Hologic Amplified Mycobacteria Tuberculosis Direct (MTD) test (San Diego,
California) and the Cepheid Xpert MTB/RIF test (Sunnyvale, California).
S, strong ; C, conditional; H, high; M, moderate; L, low; VL, very low quality of evidence