Xpert MTB/RIF testing for pulmonary tuberculosis in a central lab took twice longer than sputum smear microscopy among HIV-positive patients with suspected pulmonary TB in Durban, South Africa. The authors believe Xpert results in “operational delays that limit the test's clinical utility for diagnosing pulmonary TB” in their patients.
Sputum smear microscopy for acid-fast bacilli remains the most widely available TB diagnostic test in South Africa, where TB leads all causes of death in people with HIV. But microscopy has low sensitivity in people with HIV, while Xpert MTB/RIF improves sensitivity and has won World Health Organization endorsement.
To compare diagnostic time with Xpert versus sputum smear microscopy, researchers retrospectively reviewed findings in HIV-positive adults with suspected pulmonary TB who had Xpert TB testing interpreted at a central lab at McCord Hospital and its adjoining HIV clinic in Durban. Using medical records, they measured total diagnostic time with Xpert and standard sputum smear microscopy, defined as time from sputum collection to receipt of results by the clinician.
In the 402 study participants, median total diagnostic time measured 6.4 days with Xpert and 3.3 days with standard microscopy, a highly significant difference (P < 0.001). Compared with the standard TB test, Xpert laboratory processing took a median 1.4 days longer (P < 0.001) and result transfer to the clinic took a median 1.7 days longer (P < 0.001).
Of the 86 Xpert-positive patients who began TB therapy, 49 (57%) started treatment on the basis of clinical suspicion or standard microscopy result. In contrast, only 32 of these people (37%) began treatment on the basis of Xpert-positive results.
“In our setting, Xpert results took twice as long as [sputum smear microscopy] results to reach clinicians,” the researchers conclude. Replacing the standard test “with centralized Xpert may delay TB diagnoses in some settings,” they warn.
“Although placing Xpert at the clinical point-of-care [rather than in a central lab] may reduce diagnostic delays and improve clinical outcomes,” the authors note, “benefits must be weighed against increased technical and operational costs. In the meantime, centralized processing and reporting of Xpert testing should be streamlined to provide faster results to clinicians, and there continues to be a role for sputum acid-fast bacilli testing.”
Source: Gabriel M. Cohen, Paul K. Drain, Farzad Noubary, Christie Cloete, Ingrid V. Bassett. Diagnostic delays and clinical decision making with centralized Xpert MTB/RIF testing in Durban, South Africa. JAIDS. 2014; 67: e88-e93.