Diagnostic TB tests
The three tests are sputum smear microscopy, chest X -ray, and the CB-NAAT test. The CB-NAAT test is now being made available throughout India. The CB-NAAT test is called Genexpert in most countries outside India. There is also now the Truenat Test which is starting to be used.
Signs & symptoms of TB
Any person who has signs and symptoms suggestive of TB including a cough for more than 2 weeks, and a fever for more than 2 weeks, significant weight loss, haemoptysis (coughing blood) etc. and any abnormality in a chest radiograph should be evaluated to find out if they have TB.
This should include children with a persistent fever and/or cough for more than 2 weeks. Also children who have a loss of weight or no weight gain, and/or children who are household contacts of people who have already been diagnosed as having pulmonary TB. It is particularly important to include children who are household contacts as many of these children remain undiagnosed. There is more about TB in Children and TB Treatment for Children.
Screening for TB
People living with HIV (PLHIV), people who are malnourished, who have diabetes or cancer, and people on steroid therapy should be regularly screened for signs and symptoms suggestive of TB.
Enhanced case finding should be undertaken in certain “high risk” populations such as healthcare workers, prisoners & slum dwellers.
Enhanced case finding means having a high level of suspicion for TB in all encounters. Then excluding TB (or indeed identifying TB) using a combination of clinical queries, radiographic and microbiologic testing.
Patients to be tested
A presumptive pulmonary TB patient, is a patient with any of the signs and symptoms of TB. Most presumptive TB patients should start by having a smear examination and a chest x ray (CXR).
If the smear test is positive then this is referred to as microbiologically confirmed TB. If the bacteria that the person has is assumed to be drug sensitive then the patient should start on TB treatment with first line drugs as soon as possible.
The smear test is not always a very reliable test. So if the smear test is negative, but the chest X ray suggests TB, or a CXR is not available, then the patient should go on to have a CB-NAAT test. A CB-NAAT test should also be carried out if there is clinically a very high level of suspicion that the patient has TB.
People who are HIV positive should have a CB-NAAT test straight away.
The CB-NAAT test is a test for TB, and it also shows if the person has resistance to the drug rifampicin (Rif). There are also now starting to be some alternatives such as the TrueNat test which should be cheaper as it has been developed in India.
Resistance to Rif
If TB bacteria are detected, which means that the person has TB, then the test will also show that the person is either sensitive to Rif, is resistant to Rif, or the result may be unclear.
If the person is sensitive to Rif then this is microbiologically confirmed TB and the person should be started on TB treatment with first line drugs as soon as possible.
If the Rif result is not clear, then the CB-NAAT test should be repeated. If the result is still unclear on a second sample, then a fresh sputum sample should be collected in order that different tests such as culture can be carried out.
If the test shows that the person is resistant to Rif then the person should be referred for treatment for drug resistant TB.
If TB bacteria are not detected, then the person has probably not got TB, and an alternative diagnosis should be considered.
Drug Resistant TB
PMDT criteria is the Programmatic Management of Drug Resistant TB. As far as diagnostic tests for TB are concerned, if there is a possibility that a person has drug resistant TB, then a CB-NAAT test should be done straight away.
HIV Counseling & Testing
All presumptive TB patients should be offered HIV counseling and testing. However, the diagnosis tests for TB should not be delayed.
Sputum tests are very important in diagnosing TB, so paying attention to the detail of collecting a good sputum sample is very important. A number of studies have looked at this, and the general view is that two samples are almost as good as three samples.
This page was last updated in December 2022.
Author Annabel Kanabus
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