Diagnosing T.B could be difficult sometimes and requires more tests. Relying on finding Acid Fast Bacilli is usually not reliable in many cases. PCR for T.B also has a limitation and can not be performed on the blood. FDA approved 2 new tests for T.B. The first one is �anda-tb�. This test is relying on finding antibodies against A60 antigen complex. This antigen is found in the cytosol of typical and atypical mycobacteria. It could be IgM or IgG. The advantage of this test that healthy non-infected individuals are negative for this test even if they have positive intradermal reaction test. It is positive only in patients with active T.B infections. It can be used in blood and in CSF specimen for diagnosing T.B meningitis. It can be used for diagnosing pulmonary and extrapulmonary tuberculosis. It can be used for diagnosing typical and atypical myocobacteria. It disappears when the patient is cured, and re-appears again on relapses. The disadvantage of the test is that is can be positive in Nocardia infected individuals (since Nocardia species have A60 antigen complex in their cystosol). However, clinical examination and histologic findings are different. Also the antibodies to A60 antigen complex can be negative or weak in 5% of patient who are anergic with suppressed immunity (like in HIV infected individual). Several studies have posted the specificity of this test to be from 80-98% and sensitivity from 60-100%. (Charpin et al, Cominero et al, Wang et al). To increased sensitivity and specificity of anda-tb test, it is strongly recommended to ask for both IgG and IgM antibodies against A60 antigen complex anda-tb test
The second test is �patho-tb� test. The idea of this test is detecting mycobacteria bacilli. The specimens used in this test are sputum, pleural fluid, bronchioalveolar lavage, CSF, FNA aspirated material, lymph node fresh material, gastric and intestinal wash. The specimen is first decontaminated by specific method in the lab then filtered into a specially designed filter. When the specimen with T.B passes through the filter which is coated by anti TB antibodies, the reaction change the filter color and indicate the presence of mycobacterium or atypical mycobacterium in the specimen. The patho-tb test is about 100 times more sensitive than finding ordinary acid fast bacilli in one field
In summary, it is strongly recommended that �anda-tb� test to be requested in the blood when no tissue is available. The �patho-tb� test is recommended when tissue or fluid material (BAL, CSF, sputum, gastric or intestinal wash, or FNA aspirate) is present. Both tests carry very high sensitivity and specificity for diagnosing active T.B infection
We, in the First Medical Lab, are committed to bring the best of laboratory test to clinical practice in Jordan. Please do not hesitate to contact me for any further information
Dr Hussam Abu-Farsakh
First Medical Lab Director