What does a Positive TB Test Look Like?

Tuberculosis (TB) is a serious infection of the lungs. Mycobacterium tuberculosis is the causative agent of this contagious disease. Although it is preventable and curable, there are cases wherein the bacteria may stay dormant inside your body for years. Signs and symptoms may only appear once the disease-causing microorganism becomes active. At this point, one can easily spread the disease. For this reason, taking a TB skin test is crucial, especially if you suspect that you have been exposed to the disease.

The Mantoux tuberculin test or simply the TB skin test is the most common test used to diagnose tuberculosis. It has two parts. First, the doctor will inject a purified protein derivative (PDD) into your forearm. After 2 to 3 days, you will have to return to your doctor. He will check for swelling or bumps and give you the results.

What does it Mean if Abnormal TB Skin Test?

There are several reasons one may get abnormal test results, which includes the following:

  • You may have pneumonia wherein the bacteria have already infected the lungs. A chest X-ray will help verify this medical condition. Other symptoms may include a cough and fever.
  • You have Mycobacterium tuberculosis in your lungs. However, it is asleep and will become tubercles. These nodules may open and the bacteria will spread to the lungs and other body parts after several months or years. A chest X-ray will produce a normal result in this situation.
  • You were infected. However, your immune system has already destroyed the Mycobacterium tuberculosis. The chest X-ray will have a normal result.
  • If you had a Bacillus Calmette-Guerin (BCG) vaccine, you may also get abnormal results. BCG is a vaccine administered to reduce the risk of developing tuberculosis.
  • Abnormal results may also occur due to improper administration of the skin test.

What does a Positive TB Test Look Like?

The formation of an induration or a hard bump will occur if the test is positive. In some cases, the bump is not visible to the naked eye. You can use your fingertips to locate it. Once located, the induration is encircled and measured. Depending on its measurements, it may mean either of the following:

  • An induration with a measurement of 15 mm or higher indicates a positive result.
  • An induration with a 10 to 14 mm measurement is a positive result for individuals under the moderate-risk category.
  • An induration with a 5 to 10 mm measurement is a positive result for individuals under the high-risk category.
  • An induration of less than 5 mm is a negative result.

How to Understand the Results of TB Test?

To fully understand the results, here is a more detailed explanation of the following measurements.

  • An Induration of 5mm and more

This measurement is a positive result for the high-risk category. People under this category are the following:

  1. Individuals who are HIV positive
  2. Those who have a weak immune system
  3. Those who had an organ transplant
  4. If you had contact with someone with TB recently
  5. If you are taking any immunosuppressant drugs such as steroids
  • An induration of 10 mm and more

Individuals under the moderate-risk category have a positive result if the hard bump has a measurement 10 mm or more. Under this category are people with the following conditions:

  1. Those who inject illegal drugs
  2. Those who are working in healthcare facilities such as hospitals and nursing homes
  3. Those who are living in high-risk areas such as prisons, shelters, and refugee camps
  4. Immigrants coming from an underdeveloped country or places with high prevalence of TB
  5. Children with ages 4 and below
  6. Children living with high-risk individuals
  7. If you are working in a laboratory and was recently exposed to the bacteria
  8. If an individual has a medical condition such as diabetes, kidney failure, leukemia and other similar illnesses
  • An induration of 15 mm

All individuals with a 15 mm induration are positive for tuberculosis regardless of their background and risk factor. The presence of a blister will also indicate a positive result.

Next Steps:

  • Negative Result

If you have a negative result for you TB skin test, no additional tests are necessary.

  • Abnormal TB Skin Test Result

If your skin test result is abnormal, your doctor will order a chest X-ray to rule out pneumonia. If the chest X-ray is normal, you have several options.

  1. You can take the skin test again.
  2. You can assume that your lungs have tubercles and take antibiotics to kill the disease-causing microorganism.
  3. You can take an annual chest X-ray to check for microbial growth or activity.
  4. You can take another TB test. There are several tests used to diagnose tuberculosis such as a sputum test, an imaging test, and serological or blood test.

The last option is the best choice if you want a definite answer. You can talk with your doctor to choose the right option.

  • If the Result is Positive

If the result is positive, it does not mean that you have active tuberculosis. The skin test cannot detect if the infection is latent or active. Your doctor will then order a chest X-ray to look for white spots, which indicate the presence of infection. He may also order a computerized tomography (CT) scan for a clearer image of your lungs. Other tests such blood and sputum tests may be requested to fully diagnose your condition.

If you have latent tuberculosis, you are not contagious. The main goal is to prevent the disease. Your doctor will prescribe either isoniazid or rifampin.

If you have active tuberculosis, you are contagious. It is best to isolate yourself to avoid spreading the disease. Other symptoms may include coughing continuously, the presence of blood when coughing, chest pain, constant fever, fatigue, and sweating excessively. To treat active tuberculosis, your doctor will prescribe a combination of various medications, which includes isoniazid, rifampin, ethambutol, and pyrazinamide.

It is highly recommended to take these drugs for about 6 to 9 months to ensure full recovery, whether you have active or latent TB. Even if you are feeling better after a few days, you should still complete the treatment. After two weeks, you are no longer contagious.