TUBERCULOSIS (TB)

Successful diagnosis of TB is directly related to obtaining a good sample.

The best sample is a sputum sample after bronchial lavage. If regular sputum or urine samples

are being examined, three to six successive samples should be collected to overcome;

       ·   Poor expectoration by a patient trying to give a sputum sample.

       ·   Intermittent production of bacilli in urine.

        Diagnosis of TB is also dependent on the technique used:

1) Direct microscopy:

     Films are only positive if more than 5,000 Acid Fast Bacilli (AFB) are present in one ml of

     sputum.Scoring of a positive result should be reported by the lab in the following manner

            1-2 AFB/ 300 fields (+/-)

            1-9 AFB/ 100 fields (1+)

            1-9 AFB/   10 fields (2+)

            1-9 AFB/     1 field  (3+)

            > 9 AFB/     1 field  (4+)

2) Cultures:

     a) Conventional cultures:

                This methodology involves centrifuging the whole sample then culturing

                the deposit- after adding a cocktail of antibiotics- on several media both

                liquid and solid.Cultures are positive if more than 10 AFB are present in

                the sample. Provisional results can be given before 10 days if the organisms

                are fast growers, and after 15 days if slow growers but final results will be

                handed out in 30 days to allow for identification of the very slow growers

                and for species identification.

      b) Rapid cultures:

                 by using the BACTEC radiolabeled procedure or the MGIT fluorescin labeled

                 method. Rapid growers can be detected in 3-5 days and slow growers in 7-10 days.

                 However species identification has to be done by conventional cultures.

3) TB-DNA by PCR:

        This technique is capable of detecting one AFB/ml. It involves selectively amplifying a single

         molecule of DNA several million folds in a few hours followed by coupling of the DNA to its

         complementary primer and finally gel electrophoresis for identification. Primers are directed

         to Mycobacterium tuberculosis bacilli only.

4) Adenosine Deaminase Activity (ADA):

         This test is useful  to diagnose TB in body fluids such as CSF, Pleural & Peritoneal

         effusions. The sensitivity of the test is 100% and the specificity is 90%.

      ·      ADA activity  in pleural & peritoneal effusions

                 < 40 U/L is negative for TB

                 >70 U/L is positive for TB

         Between  40-70 U/L is suggestive, to be confirmed by repeat testing in 2 weeks.

      ·       ADA activity in CSF  > 6 U/L is positive for TB

5) Tuberculin testing:

          This involves injecting the patient intradermally with 0.1ml of 10 units purified protein

          derivative (PPD).The site of injection should be examined in 48 and 72 hours.

          If an indurated mass is formed measure its size and report the results as follows;

                  0-4 mm negative

                  5-9 mm doubtful

                  >10mm positive

           A positive result suggests exposure to TB bacilli. A negative result is of little value in
           adults,but in children it strongly suggests the need for BCG vaccination.

           False negative results can be met with in case of old age, very early TB infection,

           miliary TB, impaired cellular immunity, neoplasms, chemotherapy, irradiation,

           sarcoidosis and viral infections.           

6) Antibiogram to T.B.:

           Anti-tuberculous antibiotics tested in the lab include the first line of treatment

           (Isoniazid,Para aminosalicylic acid, Streptomycin, Rifampicin, Ethambutol) and second

           line of treatment (Capreomycin, Cycloserine, Pyrazinamide, Ethionamide).According to

           WHO guidelines, new drugs are now included such as fluoroquinolone drugs and

           aminoglycosides (as amikacin) for cases of multidrug resistant Myc. tuberculosis to

           (Isoniazid & Rifampicin).

                                                                                Interpretation of Results