Sputum Volume Predicts Sputum Mycobacterial Load during the First 2 Weeks of Antituberculosis Treatment

Abstract

dDisease severity in patients with pulmonary tuberculosis is associated with mycobacterial sputum load. To ascertain whether reduced sputum production during treatment is a useful clinical sign of improvement, we analyzed the mycobacterial loads of 5,552 sputum samples collected from 439 newly diagnosed sputum smear-positive tuberculosis patients who participated in six 14-day studies of antituberculosis treatment. Sputum volumes were categorized as low (<6 ml), medium (6 to 10 ml), or large (>10 ml), and mycobacterial load was measured by the time to positivity in liquid culture and the CFU counts on solid culture. The association of sputum volume with mycobacterial load was estimated with multiple linear regression models adjusted for repeated measures. The predictor variables were sputum volume category, treatment day, specific study , and the interaction of sputum volume category and treatment day. Mycobacterial load was significantly associated only with the day on treatment and sputum volume, which tended to decrease with ongoing treatment. With the volume held constant, each day on treatment decreased the log CFU by 0.082 (P < 0.001) and increased the time to positivity (TTP) by 1.04 h (P < 0.001). From low to medium and from medium to large sputum volumes, the log CFU/ml increased by 0.265 (P < 0.003) and 0.490 (P < 0.001), respectively, and the TTP decreased by 1.17 h (P < 0.001) and 1.30 h (P < 0.001), respectively, for a given day of treatment. The variability of the sputum load measurements increased with the day of treatment and lower sputum volumes. The significant association of sputum volume and mycobacterial load validates decreasing sputum production as a clinical sign of improvement during early antituberculosis treatment.T uberculosis (TB) remains a major cause of morbidity and mortality worldwide. In 2012, the World Health Organization (WHO) estimated that 8.5 million new TB cases and 1.3 million TB deaths occurred globally (1). In 2008, the incidence of TB in South Africa was 960 cases per 100,000 people, with an annual increase of 6.4% from 1998 to 2008 (2). TB is most commonly diagnosed by the identification of Mycobacterium tuberculosis in expectorated spot sputum specimens, either by sputum smear microscopy or by the identification of mycobacterial antigens with PCR (3). The viable sputum mycobacterial load can be measured by counting CFU on agar plates or by the time to positivity (TTP) in culture with liquid medium (4-6). Greater initial spot sputum mycobacterial burden has been linked to more extensive radiological lung involvement, worse treatment outcomes, and a higher risk of relapse (7-9), and more recently, greater sputum volume has been linked to worse treatment outcomes in HIV-infected TB patients (10).The decrease in the sputum mycobacterial burden in pooled 16-h sputum samples collected overnight is an established measure of early antituberculosis drug effects over the first 2 weeks of treatment (11), but few studies have assessed the association of sputum volumes with measu...

Application of the Multistate Tuberculosis Pharmacometric Model in Patients With Rifampicin‐Treated Pulmonary Tuberculosis

“…Sampling intervals, time points for sampling, and sample volumes can deviate within and between studies, which can be accounted for using our approach ( Supplementary Material S1 and S2 ). Patients produce less sputum as treatment progresses, which could be important to account for in future analyses . Variability in CFU is also affected, as it increases at smaller sample volumes.…”

“…Predictions of CFU were defined as: where PRED is the natural logarithm of the average bacterial number in the fast‐ and slow‐multiplying states (mL −1 ) over the sampling interval. The true sample volume was unavailable in this study and was assumed to be 10 mL for all samples, a realistic value based on reported sputum volumes . Start and duration of sputum sample collection and sample volume were specified in the dataset ( Supplementary Material S1 ).…”

“…Finally, drug effect was quantified by developing exposure-response relationships linking the PK model to the MTP (disease) model. All observed CFU data were modeled simultaneously as the natural logarithm of CFU in ml 21…”

“…where k FS 5k FS lin Á t where k D is the natural death rate in days 21 , needed to adequately describe the CFU in vitro data when assuming no nonmultiplying state. 15 The CFU declines at $70 days, which is correctly described as a shift into the nonmultiplying state in the MTP model but is described as natural death in the alternative two-state model.…”

“…The true sample volume was unavailable in this study and was assumed to be 10 mL for all samples, a realistic value based on reported sputum volumes. 21 Start and duration of sputum sample collection and sample volume were specified in the dataset (Supplementary Material S1).…”