Successful diagnosis of TB is directly related to obtaining a good sample. If sputum or urine is 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:
1-2AFB/300 fields F (+/-)
1-9AFB/100 fields F (1+)
1-9AFB/ 10 fields F (2+)
1-9AFB/ 1fields F (3+)
>9AFB/ 1fields F (4+)
a) Conventional cultures: This methodology involves 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 in slow growers but final results will be handed out in 40 days to allow for identification of the very slow growers.
b) Rapid cultures: by using the BACTEC radio labeled procedure or the MGIT fluorescing 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 culture.
3) TB-DNA by Real Time PCR:
This technique is capable of detecting one AFB/ml. It involves selectively amplifying a single molecule of DNA several million folds in few hours followed by coupling of the DNA to its complementary primer and finally detection of the TB DNA. 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
40-70 U/L is suggestive, to be confirmed by report testing in 2 weeks.
· ADA activity in CSF:
6 U/L is positive for TB