Background

A diagnostic method for assessing immunological sensitivity to Mycobacterium tuberculosis (MTB) antigens is the Purified Protein Derivative (PPD) test also referred to as the Mantoux tuberculin skin test (TST). It is predicated on delayed-type hypersensitivity (DTH) a reaction that T-cells mediate in people who have become sensitized to MTB as a result of infection or BCG vaccination. The test has its roots in Robert Koch’s discovery of tuberculin in the late 19^th and early 20^th centuries which was a major turning point in the battle against tuberculosis.

Tuberculosis is a terrible contagious disease that kills millions each year. Despite global health measures, it remains a major public health concern particularly in low- and middle-income nations. Early diagnosis of latent tuberculosis infection (LTBI) is critical for limiting its spread and delaying development to active illness. The PPD test which requires injecting a standardized quantity of tuberculin into the forearm has gained popularity because of its ease of use, low cost and large-scale screening possibilities. Individuals who have previously been exposed to MTB have a localized immunological response, marked by induration, within 48-72 hours indicating the activation of memory T cells.

Aplisol and Tubersol are the 2 PPD tuberculin antigens authorized by the US FDA. Other tuberculin antigen like RT-23 is used outside of the US. The doses for RT-23 and PPD-S formulations vary. The dose of PPD-S formulation is 5 units/0.1 mL. There are no dosage changes necessary in individuals who have renal and hepatic failure.

As per the CDC, this test is carried out by using Mantoux technique. It requires administering 0.1 mL of solution which contains 5 units of the tuberculin purified protein into inner surface of forearm by the intradermally. It must be given at least 2 inches from elbow, wrist or other areas of injection.

This test is carried out with a 1 mL of tuberculin syringe with a needle shorter than 0.5 inches and with needle bevel pointing upward. The needle must be gently introduced at an angle of 5 to 50 degrees and with the bevel visible beneath the epidermis. When a little quantity of PPD solution is administered intradermally. A wheal or skin elevation of 6 to 10 mm in diameter forms which indicate proper administration. If a wheal is not formed, the test has to be performed immediately on an alternate spot at least 5 cm apart from the first administration site.

The type IV delayed hypersensitivity response to administered tuberculin PPD antigen is triggered. This is time sensitive test. The reaction starts within 5 to 6 hours after the intradermal test is placed but it must be assessed 48 hours to 72 hours later when the maximum effect occurs. After 48 hours to 72 hours, the effect begins to fade.  Induration and erythema have been observed at the place of administration site. The diameter of the induration must be defined and measured in the mm parallel to the long axis of forearm. Erythema disease has no diagnostic use and must not be assessed. Induration is noticeable and should be assessed by observation and palpation.

Indications/Applications

PPD test is performed in individual who have elevated risk of new infection of TB and are those who have exposure to the MBT, casual or close contact with the patient who are not treated, patient who has LTBI, active respiratory TB, an elevated risk of progression of disease. It is also performed on healthcare workers like respiratory therapist, pulmonologist, who need serial test, who have high risk of exposure.

Patient who has severe immunocompromised system like organ transplant or chemotherapy, HIV infection, leukemia, lymphoma, neck or head cancer, silicosis, renal failure which need hemodialysis and who receives the treatment of TNF inhibitors are at the high risk of reactivation.

Patient who has diabetes mellitus and has systemic glucocorticoid treatment like more than 15 mg per day for 1 month or more are at 3 to 6 times of high risk of reactivation. Test must be performed in individuals who use injection drug, an elevated rate of TB. Patient who has below 85% of the ideal weight of body are at 1.5 to 3 times more at the risk of reactivation.

Reference Range

Negative Result: Induration  below 5 mm

Positive Result: Induration thresholds may vary based on risk factors like:

Above 5 mm: Positive in individuals with HIV, recent TB exposure or immunosuppression

Above 10 mm: Positive in healthcare workers, recent immigrants from high TB-prevalence areas and individuals with certain medical conditions like diabetes

Above 15 mm: Positive in individuals with no known risk factors

Interpretation

Induration below 5 mm is positive in cases like individual who is infected with HIV and had a close contact with active case

Induration of ≥ 5 mm is positive in cases like:

Immunocompromised individuals like who are having steroids like prednisone more than 15 mg for more than 1 month, TNF inhibitors or chemotherapy, Who have HIV infection, who have close contact with active TB patient, early sign of TB on chest radiography like fibronodular change, organ transplant.

Induration of ≥ 10 mm is positive in cases like:

Emigrants from the other country where the TB is endemic or rate of infection is higher than 25 per 100000, resident or employee who have high risk areas like prison, healthcare labs, mycobacteriology laboratory, injection drug user, children below 4 years, who have chronic medical disease which can elevate the reactivation of the TB like diabetes mellitus, silicosis, chronic renal failure, malignancy like cancer on neck, head, lung, lymphoma, leukemia, any history of the jejunoileal bypass.

Induration of ≥ 15 mm is positive in cases like:

Individuals who are older than 4 years with a low infection of TB

Collection And Panels

This test is performed on the surface of the arm. No sample is needed.

This test is carried out with a 1 mL of tuberculin syringe with a needle shorter than 0.5 inches and with needle bevel pointing upward. The needle must be gently introduced at an angle of 5 to 50 degrees and with the bevel visible beneath the epidermis. When a little quantity of PPD solution is administered intradermally. A wheal or skin elevation of 6 to 10 mm in diameter forms which indicate proper administration. If a wheal is not formed, the test has to be performed immediately on an alternate spot at least 5 cm apart from the first administration site.

Reading the results:

The diameter of the induration is measured after the 48 to 72 hours.

Modifying factors:

False positive results are caused because:

Any prior BCG vaccine

Infection of non-TB mycobacteria

Incorrect administration

Misinterpretation of test

False negative results are caused because:

Improper response of T cell or cutaneous anergy immunocompromise

Immunosuppressive drugs

Conditions like recent surgery, chronic renal disease, severe burn, lymphoma, severe malnutrition, sarcoidosis

MTB infection in 8 weeks of test

Old mycobacteria infection which is not detected

Children below 6 months old

Old adult

Recent bacterial, viral or fungal infections

Recently live virus vaccination like mumps, measles, polio in 4 weeks to 6 weeks of test

HIV infection

Defective material

Incorrect administration

Misinterpretation of test

Panels:

Interferon gamma release assay test (IGRA) is used as an alternative test of PPD.

References

https://www.ncbi.nlm.nih.gov/books/NBK556037/