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Is AFB a confirmatory test for tuberculosis?

Monday, 12 March 2007
Answered by: Dr. Anuj Sharma
Consultant Microbiologist
Sir Ganga Ram Hospital
Delhi
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Q. I have been diagnosed for active TB based on the positive sputum AFB test. Though my blood ESR is 13 mm/hr, and I do not have any other symptoms of TB, so is AFB a final confirmatory test for active TB? Is the AFB test positive in case of latent TB?

A.  You say that the diagnosis of TB in your case is on the basis of AFB smear alone. This is never the case. The diagnosis of TB is done in conjunction with clinical presentation (history of symptoms), detailed physical examination (to elicit the signs) along with relevant investigations. Your doctor would have suspected TB based on your symptoms, hence the test for AFB smear. The quality of the report depends on the laboratory from where the test is done and whether they are accredited, follow quality control procedures and other internationally recommended protocols. In your case, a 2 weeks history of symptoms (details not mentioned) is usually not enough, and absence of other symptoms (as you say) warrants a second opinion. You have also not mentioned the grading of the smear, since positive smears are quantified according to the number of bacilli seen per oil immersion field. AFB smear is not positive in latent TB. An x-ray chest would confirm the diagnosis if classic x-ray shadows are found. Other investigations for confirmation / ruling out TB can include Mantoux test, Quantiferron TB gold, and AFB culture.

A.  You say that the diagnosis of TB in your case is on the basis of AFB smear alone. This is never the case. The diagnosis of TB is done in conjunction with clinical presentation (history of symptoms), detailed physical examination (to elicit the signs) along with relevant investigations. Your doctor would have suspected TB based on your symptoms, hence the test for AFB smear. The quality of the report depends on the laboratory from where the test is done and whether they are accredited, follow quality control procedures and other internationally recommended protocols. In your case, a 2 weeks history of symptoms (details not mentioned) is usually not enough, and absence of other symptoms (as you say) warrants a second opinion. You have also not mentioned the grading of the smear, since positive smears are quantified according to the number of bacilli seen per oil immersion field. AFB smear is not positive in latent TB. An x-ray chest would confirm the diagnosis if classic x-ray shadows are found. Other investigations for confirmation / ruling out TB can include Mantoux test, Quantiferron TB gold, and AFB culture.

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