What Is a Positive TB Skin Test Result?
A positive result on a tuberculin skin test means that someone with active tuberculosis disease has been infected with the bacteria that causes tuberculosis. A negative result means that no one has TB disease. There are many types of tests used to diagnose TB, but only two have any real value: a culture or an enzyme immuno blotting assay (ELISA). These tests look for specific strains of bacteria that cause TB.
The Culture Test
Cultures are taken from sputum or other secretions. They’re then tested against a list of strains found in the human population. If they match, it’s a good sign that the person has TB disease. But if they don’t match, it doesn’t mean anything because there aren’t enough strains to make a diagnosis based on these kinds of tests alone.
The ELISA Test
An enzyme immuno blotting test looks for antibodies made against certain proteins in the body. The antibody is produced when the immune system recognizes a foreign substance. An ELISA test uses antibodies to detect whether someone has latent TB disease. People with latent TB disease may not show symptoms, but they do have active TB disease somewhere else in their bodies.
They need treatment before they develop symptoms again.
The Rapid Tuberculin Test
This test is similar to the skin-pustule test. The difference is that it uses a higher concentration of the tuberculin protein. That means you can get a positive result in just 48 hours, as opposed to 10-21 days with the skin-pustule test. That also means it can give a false positive.
If you think you had a pustule but don’t get one when you redo the test, you should be retested with the normal concentration that creates a pustule. If you still don’t develop a pustule, your first result was a false positive.
The Quantiferon Gold Test
This test looks for antibodies made against the protein in M. tuberculosis. Someone with active TB disease will have a positive result. The Quantiferon test is very specific, so it rarely gives false positives.
The problem is that it isn’t sensitive enough to give a positive result for someone who has only had the disease in the past but has since recovered.
Next steps after a positive test
You should have a chest x-ray and another blood test called a Lowenstein-Jensen (LJ) smear at this point. If you have no evidence of the disease on these tests, your positive test may have been a false positive. Or, it could be a case of “cough pnemonia,” which is a condition in which someone has a bacterial infection in the lungs that causes a cough, but doesn’t have TB disease.
Another possibility is that someone with latent TB disease only had it in the past, but has since completely recovered. Your doctor may decide to retest you again in six months.
If you show evidence of having active TB disease on the x-ray or LJ smear, you need treatment, even if your skin test and Quantiferon Gold results were negative.
You also need treatment if you have a past history of TB disease, and your current sputum tests positive, even if your skin test and Quantiferon Gold are negative.
If you’ve had a positive pearly penile papules treatment, your health care provider may decide to treat you for latent disease, even if all your initial tests are negative.
What to expect if you have the disease
The treatment of choice for active disease is isoniazid, rifampin, pyrazinamide, and ethambutol. Treatment for latent disease is isoniazid and rifampin (HRZE). You’ll need to take these medicines for six months for active disease or 12 months for latent disease.
Is It Preventable?
Yes. Isolate people with active disease to avoid spreading it to others and give them the appropriate treatment so they don’t pass it on. Vaccination can prevent disease in people who have a high risk of getting the infection, such as health care workers or residents and employees of nursing homes and other long-term care facilities.
How you can reduce your chances of getting it
The best way to reduce your chances is to avoid people who have the active infection — people who show symptoms. In most parts of the developed world this isn’t a major concern because TB is rare.
Sources & references used in this article:
A three-way comparison of tuberculin skin testing, QuantiFERON-TB gold and T-SPOT. TB in children by TG Connell, N Ritz, GA Paxton, JP Buttery, N Curtis… – PloS one, 2008 – journals.plos.org
Monetary versus nonmonetary incentives for TB skin test reading among drug users by CK Malotte, JR Hollingshead, F Rhodes – American journal of preventive …, 1999 – Elsevier
Comparison of two interferon-γ assays and tuberculin skin test for tracing tuberculosis contacts by SM Arend, SFT Thijsen, EMS Leyten… – American journal of …, 2007 – atsjournals.org
Interpretation of the tuberculin skin test reaction by pediatric providers by ER Carter, CM Lee – The Pediatric infectious disease journal, 2002 – journals.lww.com
Comparison of interferon-γ release assays and tuberculin skin test in predicting active tuberculosis (TB) in children in the UK: a paediatric TB network study by ARJ Bamford, AM Crook, JE Clark, Z Nademi… – Archives of disease in …, 2010 – adc.bmj.com
