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Is ELISA a good diagnostic procedure for TB?

Q: I have been suggested to go for an ELISA test for TB, as I used to develop low-grade fever in the evening with stress. The results are TB IgG is 400 sero unit and, IgM is negative. Doctor says that this does not indicate TB. What does it mean? Should I go for more tests to confirm that I have TB?

A:Based on the limited information provided you do not have TB. The ELISA test is a poor diagnostic modality for TB, having a low sensitivity and specificity. Ideally, all 3 markers, IgG, IgM and IgA should be done for serological evaluation of TB in suspected cases. Since TB is endemic in India, most of us have been exposed to TB bacilli at some time in our childhood and consequently would have some titre of IgG antibodies against TB bacilli. IgM antibodies, in high titre, usually develop in cases with a history of recent exposure to TB bacilli. In your case the ELISA for TB has no significance. A chest X-ray, and if clinically indicated, sputum microscopy and culture for TB bacilli, are better investigations for diagnosing pulmonary TB, which is the commonest form of TB. Low-grade fever is a continuous or fluctuating low fever, typically defined as not exceeding 38.5 degrees C (about 101 degrees F) at any time; it is a non-specific finding. You need a thorough clinical evaluation to find the cause of low grade fever, which can have many causes like infections (viral infections, infective endocarditis, TB, etc), autoimmune diseases, malignancy, drug side effect / interactions, etc. A diurnal variation of temperature occurs in all normal individuals with a slight peak in the evenings, and this go upto 99 degree F in some individuals. Also you have associated it with stress, so that could be the cause in your case. You do not appear to need any more investigations. You are 37 years old with a weight of 100 Kg, according to which your BMI is 33.8. This means that you are obese, and should also be monitored for your cholesterol, blood sugar and glycosylated haemoglobin.

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