The T-SPOT test is a modern blood test used to determine if an individual has been infected with Mycobacterium tuberculosis, the bacterium responsible for causing tuberculosis (TB). It functions as an aid in diagnosing the infection and is considered an alternative to the traditional tuberculin skin test. This test requires a single blood sample to assess the body’s immune response to the bacteria.
The Science of the T-SPOT Test
The T-SPOT test belongs to a class of medical diagnostics known as Interferon-Gamma Release Assays (IGRAs), which measure the reaction of immune cells to substances from the TB bacterium. In a laboratory, T-cells, a specific type of white blood cell, are isolated from the patient’s blood. These T-cells are involved in the body’s direct response against pathogens.
Once isolated, these T-cells are exposed to specially designed antigens named ESAT-6 and CFP-10. These antigens mimic proteins found on the Mycobacterium tuberculosis bacterium. They are highly specific to this bacterium and are not found in the BCG vaccine, used in many countries to prevent severe TB disease.
If an individual’s immune system has previously encountered TB bacteria, their T-cells will recognize these specific antigens and release a chemical messenger called interferon-gamma. The test measures the number of T-cells that release this substance, providing a count of TB-responsive cells in the blood.
Comparing the T-SPOT and Tuberculin Skin Test
The T-SPOT test and the older tuberculin skin test (TST) differ in their procedures. The T-SPOT test requires a single visit for a blood draw, with results processed in a laboratory. In contrast, the TST involves two appointments: an injection of purified protein derivative into the forearm skin, and a second visit 48 to 72 hours later for a healthcare worker to inspect the reaction.
This procedural difference highlights a distinction in objectivity. The T-SPOT test provides a quantitative result from laboratory equipment. The TST relies on a person measuring the size of the swelling at the injection site, which can introduce variability and subjective judgment.
An advantage of the T-SPOT test is its high specificity for individuals who have received the Bacille Calmette-Guérin (BCG) vaccine. The TST can show a positive result in people who are not infected with TB but have been vaccinated, leading to a false-positive. The T-SPOT test avoids this issue because the antigens it uses are not present in any BCG vaccine strains.
Understanding T-SPOT Test Results
T-SPOT test results are reported as positive, negative, or indeterminate. A positive result indicates the person’s immune cells reacted to the TB antigens, meaning a Mycobacterium tuberculosis infection is likely. This result does not differentiate between latent TB infection (dormant bacteria, not contagious) and active TB disease (multiplying bacteria, causing symptoms). A positive test requires follow-up evaluations, like a chest x-ray, to check for active disease.
A negative result means no significant immune reaction to the TB antigens was detected in the blood sample, suggesting a TB infection is unlikely. The test has a high sensitivity, but no test is perfect, and results are always considered alongside a patient’s medical history and potential exposures.
An indeterminate result means the test could not produce a clear positive or negative outcome. This can happen due to a problem with the blood sample or a weak immune response that prevents the test’s control measures from working correctly. Individuals with compromised immune systems may receive an indeterminate result, and in these situations, the common recommendation is to repeat the test.
Who Should Consider a T-SPOT Test?
Health organizations recommend the T-SPOT test for specific populations. One group includes individuals who have received the BCG vaccine. Because the T-SPOT test is not affected by this vaccination, it provides a more accurate assessment and avoids the high rate of false positives seen with the TST.
The test is also a practical option for people who may have difficulty returning for the second appointment required to read a TST. This includes individuals with transportation challenges, busy schedules, or those living in remote areas. The single-visit blood test is preferred for populations with low rates of returning for their TST reading.
The T-SPOT test is also considered for individuals with weakened immune systems. This includes people with HIV or patients starting treatments that suppress the immune system, such as for autoimmune disorders. In these cases, a clear laboratory-measured result is beneficial for making clinical decisions.