How common is a false positive tb blood test

There are two different types of tests used to screen for tuberculosis (TB)—a skin test using the Mantoux technique and a blood test interferon-gamma release assay. A positive test means that you have been infected with the TB bacteria. You can have a positive test even if you don't have an active and contagious form of the disease; this is called latent TB. If you're positive, your healthcare provider will pursue additional testing, including a chest X-ray and sputum culture to find out whether you have active TB—which makes you sick and is contagious to others.

Your healthcare provider will recommend the TB test that is best for you based on the reason for testing, test availability, and cost. Generally, it is not recommended to get both a skin test and a blood test. 

In addition, your healthcare provider will perform a physical exam and go over your medical history. Certain illnesses such as HIV and diabetes can put you at higher risk for TB.

Skin Testing

The Mantoux technique uses tuberculin purified protein derivative (PPD), a solution that contains certain components of the TB bacterium. A syringe with a small gauge needle is filled with the PPD solution, which is injected under the skin on your forearm.

The PPD can activate specialized immune system cells, known as T-cells, to cause a skin reaction. The degree of response is interpreted to assess whether someone has previously been exposed to the bacteria that causes TB infection.

The PPD injection is superficial, and it leaves a small, pale, raised area of skin called a “wheal” at the injection site. The wheal will eventually disappear over the course of several hours, but if it never appears, the test must be repeated. 

Results

The skin reaction reaches its peak about 48 to 72 hours after the PPD is injected. You must go back to your healthcare provider within that time period to have your reaction interpreted. Waiting too long or not long enough may cause the reading to be inaccurate.

Your healthcare provider will look for a raised, hardened area of skin called an induration. Its size will determine if you are positive or negative for TB, The parameters for interpretation depend on your risk factors for TB.

For example:

• Otherwise healthy individuals must have a larger (15 mm) induration for it to be considered positive for TB.
• Those who are immune-compromised or who have been in recent contact with someone with an active TB infection may be considered positive with a small induration (5 mm).
• An induration smaller than 5 mm indicates no TB infection.

A positive reaction will also result in redness and itching.

False-negatives and false-positives are more common with the skin test than blood tests. These results are often due to improper administration of the skin test or misinterpretation of its results. A false-negative result can also occur if you were only recently exposed to TB; a detectable immune response may not develop for eight to 10 weeks after exposure.

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Labs and Blood Tests

A blood test that can identify a reaction to TB is often preferred over a skin test because they tend to be more accurate than the skin test. That is, in part, due to the fact that it's fairly common for people to miss the required 48- to 72-hour timeframe for proper evaluation of their skin test results. 

The blood test is also more accurate than the skin test for people who have received bacille Calmette-Guerin (BCG), a vaccine for tuberculosis disease that is used in many countries with a high prevalence of TB.

If your blood test is positive, your healthcare provider may also collect a sputum sample and have it cultured to determine if you have active TB disease.

Interferon Gamma Release Assays (IGRAs) 

Blood tests used to identify signs of the immune response to TB bacteria are called interferon-gamma release assays (IGRAs). With these, blood is collected into special tubes using a needle and then sent to a laboratory for testing. There is no need for in-person follow-up for interpretation, and results are available in around 24 hours.

There are two IGRA tests approved by the Food and Drug Administration (FDA):

• QuantiFERON-TB Gold In-Tube test (QFT-GIT)
• T-SPOT.TB test (T-Spot)
  

A positive TB skin test or blood test may mean that, at some point in your life, you were exposed to and infected with TB.

In this case, other tests are necessary to determine whether you have latent TB or active TB. 

Sputum Culture

If your skin or blood test is positive for TB bacteria, your healthcare provider will do follow-up tests, including a sputum (phlegm) sample culture. Sputum is a thick fluid produced in the lungs as a result of illness. The sample will be placed in a test tube or petri dish to see if the bacteria grows. 

Mycobacterium tuberculosis (M. tb) tends to grow slowly, so it can take up to six weeks to get accurate results. 

Additionally, sputum is evaluated immediately via microscopy in conjunction with acid-fast bacilli (AFB) staining to provide a rapid assessment for M. tb or other acid-fast bacilli in the sputum. Rapid nucleic acid amplification testing (NAAT) is becoming increasingly used on sputum specimens and provides rapid results—though AFB culture remains the gold standard.

Imaging

Testing positive for tuberculosis bacteria will also necessitate having a chest X-ray. If you have active TB, the result of this test will usually be abnormal, possibly showing a shadowy area.

Occasionally, TB appears in organs outside the lungs; in those cases, CT scans, and MRIs may be used as well.

Differential Diagnoses

Some of the symptoms of TB—coughing, fever, loss of appetite, weight loss, and night sweats—are common signs of numerous other illnesses, including those that affect the lungs.

These include:

• Bacterial lung abscess (empyema)
• Chronic obstructive pulmonary disease (COPD)
• Pneumonia
• Human immunodeficiency virus (HIV) infection
• Infection with a fungus, such as in histoplasmosis
• Infection with another mycobacterium
• Lung cancer

Your healthcare provider may consider testing for these other conditions if your symptoms and preliminary tests are suggestive of these illnesses.

Frequently Asked Questions

• Could I get a false-positive result on a TB test?

  Yes. There’s a chance your skin test could come back wrong if the test isn’t administered correctly or is misinterpreted. You may also get a false-positive skin test if you were vaccinated with Bacillus Calmette-Guerin (BCG), a vaccine administered to children in countries where TB is common (it’s not given in the United States). 

  While prior BCG vaccination may result in a false-positive test, people who have had the vaccine can get infected, and not all positive test results in those who received prior BCG vaccination are false positives.

• Why would I need to be screened for TB if I have no symptoms?

  If you work in healthcare or another setting where there’s a risk of contracting or passing on TB, you may need to be screened to ensure you don’t have latent TB. This is when you show no symptoms but the infection is dormant inside your body. If it becomes active, you could pass it on.

By Mark Cichocki, RN
Mark Cichocki, RN, is an HIV/AIDS nurse educator at the University of Michigan Health System for more than 20 years.  

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