Successful control of tuberculosis (TB) depends on early and effective prevention of the transmission of Mycobacterium tuberculosis (MT) from infectious patients. Sputum sterilization is a cardinal index of treatment success and low patient infectivity, and is used to establish the time for airborne isolation in outpatient and inpatient settings. In countries with greater resources, documentation of culture conversion is currently recommended before completion of anti-TB treatment.1,2 Despite the discordance between smear results and culture results (described below), smear microscopy is faster, simpler, and less expensive than culturing, and has thus been widely used for diagnosis and treatment in resource-limited areas.1

Some studies reported that the number of acid-fast bacilli (AFB) decreases rapidly after starting treatment and that 80–85% of TB patients become non-infectious after about 2 weeks.3–5 However, Wang et al.6 estimated that more than 50% of TB patients probably remained infectious after 2 weeks of treatment. This highlights the importance of regular smear and culture monitoring until three consecutive samples are negative in order to establish the time needed for respiratory isolation and to provide proper protection for uninfected contacts. This issue had been previously noted by Clancy.7 Epidemiological studies indicated that the proportion of TB patients who remain smear-positive after 2 months of treatment can be greater than 20% (3.3–25.3%).6–13 Other evidence indicates that failure of smear and/or culture conversion in the second month of TB treatment is a predictor of patient infectivity and treatment failure.6,8–11,14 Thus, identification of the risk factors is very important for TB control policies and for allocation of public health resources. Several studies6,13,15–18 have reported that male gender, diabetes mellitus, smoking, radiologically extensive disease, cavitation, smear grade, and other factors increase the risk for TB infectivity. However, some of the results of these studies were inconsistent due to differences in methodologies, such as the use of smear conversion rather than culture conversion. In particular, previous research has indicated that there can be a discordance between 2-month smear and culture conversion results, and that up to 30% of TB patients who become culture-negative remain smear-positive.19–21 This is due to the presence of nonviable AFB, nontuberculous mycobacteria colonization, or false-positive results.19–21 A recent systematic review and meta-analysis performed by Horne et al. concluded that both smear and culture methods have low sensitivity and specificity for prediction of treatment failure and relapse.22 This position has been argued by some23 in relation to issues concerning methodology, however, Su et al. demonstrated that the limited predictive value of the 2-month smear in culture conversion was due to the significant impact of some clinical factors, beyond smear status, such as cavitation, rifampicin resistance, and use of a directly observed therapy (DOT) strategy.24

The disappearance of AFB from smears and cultures is the most widely accepted determinant for establishing the length of isolation and treatment of patients with pulmonary TB, although caution and clinical judgment must be used in the interpretation of these results, mainly in relation to smear interpretation. The present study was carried out to determine the time to smear and culture conversion and to identify potential predictors for delayed smear and culture conversion in patients with pulmonary TB.

A total of 136 patients were retrospectively enrolled. The median age was 41.0 (IQR 18.0) years, and 75.0% of the patients were male. Table 1 shows the socio-demographic characteristics and comorbidities of enrolled patients at baseline. All patients had sputum culture-proven pulmonary TB and 118 patients (86.8%) were smear-positive.

Median smear and culture conversion time was 46 (range 1–343) days and 46 (range 1–127) days, respectively. After 2 months of treatment, 25.4% (30/118) and 27.2% (37/136) of patients remained smear- and culture-positive, respectively. Among the 30 patients with delayed smear conversion, 6 achieved culture conversion. Table 2 shows other characteristics of TB in the enrolled patients.

Regular sputum smear and culture monitoring during anti-TB treatment allows assessment of sputum conversion, an important issue for therapeutic planning and counseling of TB patients,28 especially in countries such as Portugal29 with an intermediate incidence of TB.

In the present study, we independently analyzed smear and culture results of patients with pulmonary TB. The median times to smear and culture conversion were above one month for both groups, although there were wide and variable ranges which demonstrate the influence of different factors. Current smoking and bacillary load (smear grade and colony count) were associated with long conversion times for both examinations. Male gender was associated with longer smear conversion time and unemployment, bilateral radiological involvement, and cavitary disease were associated with longer culture conversion time. The range of smear conversion time was greater than the range of culture conversion time, presumably due to the presence of nonviable bacilli.20,24

Our results indicated delayed smear and culture conversion in 25.4% and 27.2% of TB patients, respectively. Previous studies have reported varying conversion times in TB patients, and this variation is due to differences in geography, baseline smear status, methodologies, and statistical analysis.6,8–16,18,24,30–32 In the present study, we found that delayed smear conversion was independently associated with age older than 50 years, male gender, and higher pre-treatment smear grade.

Previous studies have shown that lack of smear conversion is more common in older patients,11,15,18,33,34 due to their increased incidence of physical disabilities, the non-efficacious bacilli clearance due to decreased immune response, and delay in seeking diagnosis and care. The potential role of some comorbidities, such as those affecting anti-TB drug absorption and metabolism, may also play a role.

Studies of the effect of gender on sputum conversion are contradictory. In the present study, we found that male gender was associated with delayed smear conversion. Our results agree with those of Rekha et al., who speculated that this was due to the greater prevalence of alcohol consumption and smoking by men.18 In our study, however, we found no effect of gender on alcohol consumption and smoking (data not shown).

We found that patients with high pre-treatment smear grade (3+/4+) were less likely to convert than patients with low pre-treatment grade (1+/2+). Previous studies have reported similar results.6,8,11,35 Rieder reported that smear conversion at 2 months was 90.9%, 77.9%, and 61.7% in patients with initially weak, moderate, and strong positivity, respectively.8 Other studies reported that persistent smear positivity was associated with higher pre-treatment grade due to the initially high mycobacterial burden.36,37

Our univariate analysis indicated that positive culture status at 2 months was associated with unemployment, HIV positivity, bilateral radiological involvement, cavitation, and higher colony count. However, our multivariate analysis indicated that only bilateral radiological involvement and higher colony count were independently associated with that outcome.

Our data indicated that unemployed TB patients had higher prevalence rates of current alcohol consumption, smoking, and HIV positivity (data not shown), so these are potential confounders. Previous evidence has shown that active and passive smokers have an increased risk of contracting active TB compared to non-smokers,38–41 but there is insufficient data on the association between smoking and outcome variables, such as culture conversion.38–41 Our univariate analysis indicated that HIV positivity was associated with delayed culture conversion, but HIV status was not significant in the multivariate analysis. This is in line with some previous studies which have indicated that HIV status does not negatively influence culture conversion, since its positivity has been associated with lower bacillary load expressed, as in our study, by a lower prevalence of cavitary disease in these patients (data not shown).18,30,33,35,42

Our results indicated that bilateral radiological involvement and higher colony count were independent risk factors for delayed culture conversion, due to the high baseline bacillary burden of those patients. In contrast with previous studies,6,11,24,28 our results indicated no relationship of cavitation with delayed smear or culture conversion, although its presence has been significantly associated with a longer time to culture conversion. This discrepancy could be due to differences in methodologies, populations, our small sample size, and/or the retrospective design of our study.

In this study, factors influencing smear and culture conversion were different. Consequently, we can speculate about the importance of evaluating each mycobacterial examination separately. Su et al. performed a prospective analysis of 371 TB patients and reported that the predictive value of the 2-month smear in culture conversion was limited because it was highly influenced by clinical factors, such as initial smear results, cavitation, rifampicin resistance, multidrug resistant TB strains, and DOT.24 Similarly, not all of our patients presented simultaneously with delayed smear and culture conversion.

Knowledge of the risk factors associated with delayed culture conversion assists identification of the highly infectious patients who require the most medical resources and prolonged respiratory isolation, and necessitates a cautious interpretation of sputum smear results. Consequently, some authors22,24 advocate use of a new measure, such as a biomarker with high accuracy and short yield time, rather than culture results, for assessment of treatment response and infectivity during anti-TB treatment.

The main limitations of the present study are the small sample size and its retrospective design, with the consequent data missing and subjective evaluation of some features. The authors tried to minimize these limitations by careful review of all clinical files and available examination results.

In conclusion, our analysis showed delayed smear and culture conversion in about one third of patients. Older age, male gender, and higher bacillary load were independently associated with delayed smear conversion; bilateral radiological involvement and higher colony count were independently associated with delayed culture conversion. We suggest that intensified treatment and precautions against transmission should be especially considered for TB patients with these risk factors, allowing the optimization of national TB control measures.

The authors have no conflicts of interest to declare.