HOW IS TB DIAGNOSED?
(A) PULMONARY TB (TB OF THE LUNGS):
Suspecting TB is the first crucial step.
Once ‘long duration of cough, low grade fever and weight loss’ have given rise to suspicion of TB, the sick person is asked to cough deeply and forcefully and bring out his spit (called sputum), which is tested under a microscope.
Presence of the causative TB germs in his sputum confirms the diagnosis of TB.
Such a patient is said to be sputum positive.
However, not all patients suffering from lung TB emit germs. In about 50% cases, even repeated sputum tests would fail to demonstrate any germs. In these sputum negative cases, the diagnosis is based upon indirect evidence – presence of abnormal shadows in Chest X-rays.
TESTS FOR LUNG TB
For all practical purposes, there are only 2 tests for the diagnosis of lung TB:
1. Sputum smear microscopy for AFB:
There is only one definite proof of lung TB – a sputum positive report; we see causative germs in the person’s spit under a microscope.
a) Sputum positive report clinches the diagnosis.
b) It also serves as a warning that the patient is infective.
c) He must observe precautions (do’s and don’ts) to check transmission.
d) His treatment deserves topmost priority, as it will stop TB at source.
TB casts shadows of various shapes and sizes in a chest X-ray. A cloudy shadow with central clearing, a cavity, fibrosis, calcification etc. do suggest TB, especially if located in the upper portions of lungs.
However, no pattern of shadows is absolutely typical of TB.
Other chest diseases (like pneumonia, fungus, cancer etc.) can also produce similar shadows.
Therefore, diagnosis by X-ray alone is unreliable.
Besides, X-ray is indirect evidence. Sputum test remains the gold standard test. Obviously, every patient doesn’t emit germs. If no germs are detected in the sputum, one has to rely on chest X-ray for diagnosis of sputum negative TB.
A series of chest X-rays is much more helpful than a single picture.
Proper treatment makes sputum free of germs. It also clears / shrinks / fades the TB shadows – hence sputum test and X-ray have value in the follow up of patients.
A thumb rule:
A normal chest X-ray rules out lung TB in a sputum-negative chest symptomatic (a person having long-standing cough and fever as in TB but germs never detected in his sputum).
3.Sputum culture and sensitivity test:
First we grow TB germs on a suitable medium in the lab and then pour various drugs over them to observe which ones effectively kill the germs.
Even though a very sensitive test, it takes 6 weeks to grow the bacilli on traditional L J Medium and another 6 weeks for drug-sensitivity evaluation. Obviously, no patient can afford to wait for 3 months for his routine diagnosis, which can instantly be clinched by sputum smear tests.
Newer fast track version – radiometric essay test – takes less time. But a single test costs Rs. 3000 – 6000 (thrice the cost of TB treatment) in a private lab in India. Besides, the test is often unavailable / inaccessible for public. Further, reliability of lab reports is always under a cloud. In India, it is reserved only for evaluating suspected drug resistant cases and that too in theory.
Culture tests are routinely conducted in the affluent West.
(B) EXTRA-PULMONARY TB (TB IN ORGANS OTHER THAN LUNGS):
TB can occur anywhere in human body – from head to toe – for example in lymph nodes, bones, joints, spine, brain, intestines, liver, kidney, genitals and skin etc
In all these cases, as a rule, it is not possible to demonstrate the presence of TB germs.
1. ‘Biopsy and Histopathology’ form the basis of diagnosis in extra-pulmonary TB.
Taking Biopsy or doing an FNAC (Fine Needle Aspiration Cytology) – aren’t these procedures blind and hazardous?
No, not any more!
A well-equipped modern surgeon can practically reach and pinch almost any organ within the abdomen or chest with the help of an Endoscope or a laproscope under relative safety of visual guidance from X-ray screening, ultrasound, CT Scan or MRI etc.
But please remember, a few extra-pulmonary cases will also have an associated element of lung TB, wherein sputum smear tests and chest X-ray will help clinch the diagnosis conveniently, obviating surgical intervention.
2. Some new fancy tests:
PCR (Polymerase Chain Reaction), Eliza, Finger Printing, GLC (Gas Liquid Chromatography), MS (Mass Spectrometry), HPCL (High Performance Liquid Chromatography), ADA Estimation etc. are still research tools – some exhibiting more promise for future than others. They surely remain beyond the reach of the third world labs and masses. For details, kindly look elsewhere – may be at literature emanating from the developed world.
3. Tuberculin skin test:
It has some diagnostic value in small children; the smaller the child, stronger the positive skin reaction – the higher the probability of active disease.
In Indian environment, it is almost of no practical value in adults and is rarely employed.
Positive test is viewed to indicate presence of infection rather than active disease.
A positive test doesn’t confirm TB sickness.
A negative test doesn’t rule it out.
However, in a few cases, a physician has no option but to resort to a presumptive diagnosis and start TB treatment