Background: Sputum microscopy for acid-fast bacilli (AFB) is the commonest diagnostic method for pulmonary tuberculosis (PTB) in developing countries. The method is reported to be less sensitive in human immunodeficiency virus (HIV) positive compared to negative patients. We determined the bacillary density in sputum of smear-positive PTB patients and related it to the patients' HIV status, CD4 cell count, clinical and demographic characteristics.
Methods: Three sputum samples per patient were examined using microscopy before initiating therapy. The AFB density was graded according to World Health Organization recommendations. The smear with the highest density was used. High bacillary density was defined as >10 AFB/field. HIV status and CD4 cell count were determined according to the national guidelines.
Results: Of 844 patients, 433 (51.3%) were HIV-positive. High bacillary density was significantly less common among HIV-positive (39.0%) than -negative (75.7%) patients (prevalence ratio 0.52; 95%CI 0.45-0.59, P < 0.0001). Among HIV-positive patients, the proportion of those with high bacillary density increased progressively with CD4 cell counts (P = 0.003).
Conclusion: HIV is associated with lower AFB concentration in sputum. The AFB density falls with falling CD4 cell count. Microscopy for AFB in sputum may be less sensitive in diagnosing PTB when HIV infection is present, especially in severely immunocompromised patients.