Testing & diagnosis of TB in India - CB-NAAT, Smear, CXR

Testing & screening for Pulmonary TB

Testing for pulmonary 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.

Children with a persistent fever and/or cough for more than 2 weeks, 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 must also be evaluated for TB.

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.

Diagnostic TB tests

There are a number of diagnostic TB tests currently available. Three of these are now recommended in the new National TB plan to be used for adults who might have pulmonary TB.

The three tests are sputum smear microscopy, chest X -ray, and the new 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. The new recommendations are contained in the Revised Technical & Operational Guidelines for TB Control in India. Whether the new recommendations are being followed does depend on how quickly different states are complying with the new strategic plan.

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, and the patient should start on TB treatment 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.

CB-NAAT result

CB-NAAT 4 module machine © GHE

CB-NAAT 4 module machine © GHE

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 basic TB treatment 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 to those people who can recommend the treatment that should be provided for people who are Rif resistant.

If TB bacteria are not detected, then the person has probably not got TB, and an alternative diagnosis should be considered.

PMDT criteria

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.

Binocular microscopes are used for testing TB samples and the diagnosis of TB in India

Binocular microscopes are used for testing TB samples and the diagnosis of TB in India

Sputum Samples

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.

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