This study showed that of those found to have a negative result for AFB, a significant proportion (27.2%) had sputum culture positive for MTB. Therefore our data indicate that smears did not detect PTB in a very large proportion of patients. Sputum culture being the gold standard for the diagnosis of Tuberculosis disease [9
] shows that sputum smear is not a very sensitive tool in the diagnosis of PTB. This has been shown by other studies where sensitivity has been described to be between 51% to 53.3% [10
]. One of the reasons for low sensitivity is reported to be due to the fact that 104
/ml are required for AFB to be seen using smear microscopy [4
Although the gold standard for the diagnosis of Tuberculosis involves the isolation and identification of Mycobacterium Tuberculosis (MTB) using cultures [9
], the cost and facilities of doing cultures are prohibitive in most developing countries. Sputum smear microscopy remains the main diagnostic tool for PTB that allows initiation of treatment and monitoring of patient progress [11
]. As sputum smear and microscopy is not a very sensitive tool in the diagnosis of PTB, presumptive diagnosis is usually made based on an algorithm of clinical and radiological criteria. This is commonly termed as AFB negative PTB [9
]. In some cases when sputum smears are negative but the patient has clinical features highly suggestive of PTB, broad-spectrum antibiotics are recommended for at lest 10-14 days and sputum smears repeated. If the patient's condition does not improve while sputum smear remains negative, a chest radiograph is done and if found to be abnormal, a presumptive diagnosis of PTB is made and the patient is started on anti-Tuberculosis treatment as AFB negative PTB [9
]. In this study patients whose sputum smears were AFB negative, were evaluated using the above algorithm by the treating doctors at the clinics or hospital. A presumptive diagnosis of AFB sputum smear negative PTB was made in 41.8% (173) of all study subjects, and patients were started on anti-TB treatment as recommended by the Tanzania NTLP. The remaining 58.2% (240) patients were assumed to have other forms of respiratory diseases and were treated accordingly.
Less than half (38.1%) of those who were presumed to have active TB and started on Anti TB actually had TB by sputum culture results. More than 60% of these patients they received 8-months of treatment despite having a negative culture results. This is similar to what has been reported before in Malawi were it was reproted that 40% of smear negative had TB confirmed microbiologically after taking Broncho alveolar larvage [14
]. The treatment of individuals without tuberculosis adds to the cost of the TB programs in most developing countries. Likewise about 48% (61) of patients who had active tuberculosis by the results of sputum culture were missed and they received inappropriate treatment, leaving them vulnerable to developing severe disease as well as remaining source of TB infection in the community.
The current diagnostic algorithm leading to the establishment of the diagnosis of AFB smear negative PTB is inefficient; it over-diagnoses PTB and misses a lot of people with active disease. Instituting a more sensitive diagnostic tool will prevent the unnecessary cost of treating individuals who do not have TB and at the same time it will prevent the further spread of TB. This emphasizes the need of culture and the need of further research in order to identify a better diagnostic tool for diagnosis of AFB negative PTB.
In an attempt to improve on the diagnostic algorithm, the study looked at the clinical presentation of the patients to identify clinical laboratory and radiological features that are associated with smer negative PTB and which can be used to predict PTB in patients with symptoms suggestive of PTB. A multivariate analysis showed the following features to be highly predictive of AFB negative but culture positive PTB; low eosinophil counts, a mixed type of anaemia and the presence of cavities on chest radiographs. Low eosinphil seems to be an incidental finding Further studies have to be done to confirm this findings
Limitation in the current study is the inclusion of patients with cough of more than two weeks in which there may be inclusion of patients with simple chest infection that sometime may be complicated with cough for 2-3 weeks. This may be a selection bias that may explain the low sensitivity and specificity of the diagnostic algorithm.
Another limitation is the method of sputum delivery, which is delivered by the patient himself, may have affected the results as some might bring saliva.
We could not be certain that the algorithm was followed at all times because resechers were not involved in the management of these patients rather we evaluated the treatment gien to patients by the attending clinicians. In Tanzania National TB and Leprosy programme is well organized and the algorithm is well adhered by the District TB and Leprosy Coordinators and all workers of the NTLP who were the attending clinicians in this study