The diagnostic accuracy of urine-based Xpert MTB/RIF in HIV-infected hospitalized patients who are smear-negative or sputum scarce

Abstract

Background: Hospitals in sub-Saharan Africa are inundated with HIV-infected patients and tuberculosis (TB) is the commonest opportunistic infection in this sub-group. Up to one third of TB-HIV co-infected patients fail to produce a sputum sample (sputum scarce) and diagnosis is thus often delayed or missed. We investigated the sensitivity of urine-based methods (Xpert MTB/RIF, LAM strip test and LAM ELISA) in such patients.

Methodology/principal findings: 281 HIV-infected hospitalised patients with clinically suspected TB provided a spot urine sample. The reference standard was culture positivity for Mycobacterium tuberculosis on ≥1 sputum or extra-pulmonary sample. MTB/RIF was performed using 1 ml of both unprocessed and, when possible, concentrated urine. Each unconcentrated urine sample was also tested using the Clearview LAM ELISA and Alere LAM strip test. 42% (116/242) of patients had culture-proven TB. 18% (20/54) were sputum scarce. In sputum-scarce patients, the sensitivity of urine MTB/RIF and LAM ELISA was 40% (95%CI: 22-61) and 60% (95%CI: 39-78), respectively. Urine MTB/RIF specificity was 98% (95%CI: 95-100). Combined sensitivity of urine LAM ELISA and MTB/RIF was better than MTB/RIF alone [MTB/RIF and LAM: 70% (95%CI: 48-85) vs. MTB/RIF: 40% (95%CI: 22-61), p = 0.03]. Significant predictors of urine MTB/RIF positivity were CD4<50 cells/ml (p = 0.001), elevated protein-to-creatinine ratio (p<0.001) and LAM ELISA positivity (p<0.001). Urine centrifugation and pelleting significantly increased the sensitivity of MTB/RIF over unprocessed urine in paired samples [42% (95%CI: 26-58) vs. 8% (95%CI: 0-16), p<0.001]. Urine MTB/RIF-generated C(T) values correlated poorly with markers of bacillary burden (smear grade and time-to-positivity).

Conclusions/significance: This preliminary study indicates that urine-based MTB/RIF, alone or in combination with LAM antigen detection, may potentially aid the diagnosis of TB in HIV-infected patients with advanced immunosuppression when sputum-based diagnosis is not possible. Concentration of urine prior to MTB/RIF-testing significantly improves sensitivity.

Figure 1. Study flow diagram.

^§Both sputum and non-sputum samples (e.g. blood, pleural or pericardial fluid) were collected by attending clinicians for liquid TB culture. M.tb culture positive patients had at least one sputum or non-sputum sample liquid culture positive, while m.tb culture negative patients had at least one (usually 2 or more) samples liquid culture negative *Results using a grade 2 cut-point for the urine LAM strip test are shown.

Figure 2. Venn diagram showing the proportions of patients diagnosed by urine MTB/RIF and/or urine LAM ELISA in sputum scarce m.tb culture positive patients.

6 patients were both urine MTB/RIF and LAM ELISA positive; 6 patients were urine LAM ELISA positive, but MTB/RIF negative, while 2 patients were urine MTB/RIF positive, LAM ELISA negative.

Figure 3. Urine-based MTB/RIF-generated C_T values correlates with other markers of bacillary burden.

In m.tb culture and urine MTB/RIF positive patients, MTB/RIF-generated C_T values were correlated with (A) urine LAM ELISA concentration, (B) urine LAM strip grading, (C) either sputum or non-sputum liquid culture time-to-positivity (TTP), (D) only sputum liquid culture time-to-positivity, and (E) smear-grade in smear positive patients.