Investigations Diagnosis of TB

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Sputum (phlegm) smear examination : It is the test of first choice and is donefor detecting the TB germs (AFB – Acid Fast Bacilli). It can be done properly only in standardized laboratories. Normally, the patient should give three (early morning) sputum samples on which staining and microscopy can be done to detect the TB germs.

  • Sputum should be collected in a disposable, wide-mouthed and clean container (leak proof) with a tight-fitting lid. Patient should inhale (2-3 times) and then cough out deeply from the chest to bring out the sputum which is deposited in the container. Patient should not give the saliva or nasal secretions. The sputum sample should be examined visually to check its quality (i.e. thick yellowish purulent / blood containing sputum) and amount (at least 2 ml).
  • The sputum samples can be further tested for confirmation of active disease by culturing the sample on special culture media (LJ media) at specific temperatures. As the TB germs grow slowly, the test is usually read after 4 weeks to look for any bacterial growth in the culture medium. This is a confirmatory test for TB germs in the body.
  • The growth on culture media can be further subjected to sensitivity testing to check for drug resistance to any anti-tubercular drug. It takes another 4-6 weeks after the initial culture report to get the sensitivity pattern. If the bacteria grows in the presence of the anti-tubercular drug in a culture medium, then that means they are resistant to that particular drug. If they do not show any growth, it signifies that they are susceptible to those drugs.
  • In cases where the TB germs are present in a small number, the germs can be demonstrated by a special type of technique called the : `Fluorescent Microscopic Technique‘.
  • In cases where the lungs are not involved and sputum is not produced, e.g. in an enlarged lymph node, a needle biopsy can be used to carryout the above procedures as well as study the characteristic histopathology.
  •   Similarly, for TB of the intestine, the stool sample can be used for smear and culture examination.
  • For kidney TB, urine can be used for demonstration of bacteria. At least three early morning urine samples should be examined
  • For brain – The fluid, taken from the spinal cord coverings (cerebro-spinal fluid or CSF) can be used.
  • For others, like skin, bone, joints, abdomen, etc, appropriate  tissue biopsy / body fluid can be taken to detect TB bacteria as well as study the characteristic histopathology.
  • X-Ray of the chest is also an important tool for diagnosis of TB.

o        Though no signs are specific for TB, in presence of a strong clinical suspicion of a doctor or a positive sputum smear, it is diagnostic for TB.

o        The X-ray picture can also be used to assess the extent of lung involvement and to monitor the progress (improvement / deterioration) of the disease in patients who do not produce any sputum.

o        Radiology (X-rays) of other areas like bones, joints etc. are also useful in monitoring the course of disease.

  • Ultrasound / CT Scan / MRI Scan can be used to assess:

o        Fluid in the chest cavity, heart and abdominal cavity

o        Liver and kidney

o        Enlarged lymph nodes in the abdomen and chest

o        Brain and other vital organs.

  • Other tests for TB include:

o        ELISA test – It is a blood test done to detect antibodies against TB. It is not a very specific test to detect active disease (it only denotes previous infection). It is used as an adjuvanttest to check for any previous exposure to TB bacilli. It has no role in confirming or ruling out (active TB) disease.

o        PCR – An expensive but sensitive test, it takes about 1-3 weeks for the result and may also help in diagnosing TB in a smear negative / extra-pulmonary case. According to WHO both PCR and ELISA are expensive and often unreliable tests for the diagnosis / monitoring of tuberculosis.

o        BACTEC – It is also an expensive but sensitive test. The test results are usually available in 1-3 weeks in the form of a radiometric culture (measures CO2 liberated by TB germs).

  • PCR and BACTEC can be done on sputum or other tissue biopsy / body fluid sample.
  • Other non- specific tests include:

o       Increased white cell count

o      Raised E. S. R.

o      Mantoux or the tuberculin test is again a non-specific test for tuberculosis which only denotes previous infection (exposure) with TB bacilli. It is useful in diagnosing the disease in children less than five years of age.

Method: The Mantoux test is done by injecting 0.1 ml of a special type of solution into the skin of the forearm of the suspected person. The test is read after 48-72 hours. A positive test is indicated by a swelling (induration) on the injection site of a minimum of 10 mm size. The positive response indicates that the person has at sometime been infected with TB. A negative mantoux test does not rule out TB.

“False positive”test may be seen in people who have been vaccinated against Mycobacterium tuberculosis with BCG and in those infected with non-tuberculous mycobacterium.

Conditions suppressing the Mantoux test response (i.e. giving a “false negative” test) include:

o        Malnutrition

o        Severe infections

o        Viral infections e.g. measles, chickenpox

o        HIV infection

o        Drugs – anticancer drugs, steroids

Tuberculin test is of value in the following cases:

  • While investigating children suspected of having TB
  • For statistical purposes

It is used as an additional tool in patients where the diagnosis is uncertain even after other specific investigations for tuberculosis.