A cross-sectional study with prospective enrollment was conducted at Muhimbili National Hospital (MNH), a university teaching and national referral hospital, and at out-patient tuberculosis clinics at the Infectious Disease Clinic (IDC), Mwananyamala, Temeke and Ilala district hospitals, from September 2000 to December 2000. All these hospitals are located in the city of Dar es Salaam. The city accounts for over 26% of all new tuberculosis cases reported each year in Tanzania .
Adult male and female patients aged 18 to 75 years, presenting with chronic cough (≥2 weeks); who were three times sputum smear-negative for AFBs (ZN stain); and who gave a written informed consent to participate in the study and for HIV testing were included into the study. Patients with known tuberculosis or who had PTB in the past, on anti-TB for treatment or prophylaxis; those with known chronic respiratory diseases (e.g. bronchial asthma, chronic obstructive pulmonary disease, bronchiectasis), those with misplaed HIV results, contaminated cultue results and those with heart failure were excluded. The study protocol was approved by the MUHAS Ethical Review Committee.
A detailed medical history and physical examination was done by a study clinician and the findings were recorded on a clinical record form. The investigators did not interfere in the treatment of these patients. The treatment centre followed the diagnostic algorithm for smear negative. Laboratory tests included a complete blood count (coulter counter model, manufacturer, city and country) which included estimation of haemoglobin, red blood cell count and indices; and white blood cell count both total and differential. A peripheral blood smear for assessment of red cell morphology was also made. Erythrocyte sedimentation rate (ESR) was set using the Westergren method within 2 hrs of drawing blood.
HIV testing was done according to the Tanzania National AIDS guidelines. Each patient received pre- and post-test counseling and the HIV test was done using a dual ELISA algorithm. Sera which were non-reactive on first ELISA were considered HIV antibody negative, and those reactive on first ELISA were retested by a second ELISA based on a different test principle. Sera reactive on both ELISA tests were considered HIV-positive. Samples with discordant test results were confirmed by Western blot (WB) and western blot interpretation was done according to the WHO criteria .
Patients who came to the clinic with symptoms suggestive of PTB had their sputum examined. Those who were three times smear negative were consequently selected and asked to bring one more sputum sample which was sent to the Tuberculosis Reference Laboratory at MNH for AFB culture (Löwenstein-Jensen culture media).
Smears were considered positive if AFBs were seen on smear from any of the three sputum samples. Patients found to have sputum smear positive were treated for tuberculosis according to the National Tuberculosis and Leprosy program treatment guidelines. Those found to have sputum smear negative for AFB and who consented were enrolled into the study. A chest x-ray was ordered for those who were found to be HIV positive. If the chest x-ray results were abnormal, the patient was considered to have sputum smear negative PTB, and started on anti-TB medications. The rest were treated with broad-spectrum antibiotics. All enrolled patients were requested to stay at the clinic for a month for follow up. Two weeks later, patients on broad spectrum antibiotics were evaluated again by doing sputum smear and chest radiograph. Those found to have smear negative sputum but had symptoms still suggestive of PTB were treated as smear negative PTB; others were treated accordingly.
Two weeks later we came back to review treatment of clinician of which others were given ant TB and others were treated for other respiratory problems. Because they followed NTLP diagnostic algorithm, all of them had a chest X-ray done, and all of them were reviewed and reported using a structured format by two independent radiologists. In case of disagreement in their initial independent reporting, they reviewed the radiographs together and resolved the disagreement by consensus.
Sample size and data analyisis
This study was part of another study on that aimed to investigate/examine sputum smear negative but culture positive PTB the association with HIV.
Assuming a sensitivity of 50% and specificity of 75%, a sample of size 413 subjects would give a 95% confidence interval of plus/minus 0.048% for sensitivity and plus/minus 0.042% for specificity. This is a reasonable amount of precision for the given sample size.
Data were analyzed using Statistical Package for Social Sciences (SPSS) and EPI Info. Pearson chi-square test was used for comparison of categorical data and a student t-test was used for continuous data. Logistic regression analysis was applied and the direct effects of the predictors were assessed by their 95% confidence intervals. A p-value of < 0.05 was considered to be statistically significant.
Sensitivty and specificity of the diagnostic algorithm was calculated using the following formulas:
Positive and negative predictive value was calculated using the following formlas