Tuberculosis remains a major public health concern and a leading cause of morbidity and mortality worldwide, especially in developing countries. Tuberculosis is a fatal bacterial infection caused by bacterium ‘Mycobacterium tuberculosis’, a highly contagious droplet infection, which primarily affects the lungs. However, it can affect any area of the body, which includes bones, joints, central nervous system, etc. The treatment for tuberculosis is available and is effective.

Although the overall incidence and prevalence of tuberculosis have been declining, the incidence of multidrug-resistant tuberculosis is steadily rising. The Asians and Hispanics account for more than half of the new cases of tuberculosis, with the highest incidence in India, China, Indonesia, Pakistan, Nigeria, and South Africa. However, in the United States, recent trends show a significant decline in this infection.[1]

Active and Latent TB Infection

 A person with active infection usually presents with symptoms of the part affected and constitutional symptoms such as unexplained weight loss, fever, fatigue, loss of appetite, and night sweats. The latent TB, however, is asymptomatic and non-infectious.

Early diagnosis of active TB is very important to manage the disease in time and prevent its spread. The latent TB infection is non-infectious and asymptomatic, with a significant worldwide prevalence (33%). Since this population carries a risk of reactivation in immunocompromised states and progress to an active TB disease, which is symptomatic and highly contagious, latent TB is an important public health issue. The LTBI progression risk to active disease is maximum in the first two years after exposure. The progression Detection and treatment of latent TB cases is important for tuberculosis control and to reduce the disease burden.

The LTBI progression risk declines with age as the immunity increases with age. The risk of progression in infants is 50%, which declines to 1% to 2% by the age of 10 years.[2]

TB Screening Tests

TB infection can be demonstrated by two screening tests, namely:

• PPD Skin test (Mantoux test/tuberculin skin test)[3]
• IGRA (interferon-gamma release assay)[4]

Both these tests evaluate cell-mediated immunity, which usually occurs when the person has been exposed to TB bacteria. The skin reaction is the response mediated by T lymphocytes (cell-mediated immunity). The positivity of these tests, however, does not distinguish between latent or active tuberculosis. Therefore, symptom assessment and further testing (chest radiograph, sputum test for acid-fast bacillus, CT scan) and are essential to look for an active infection.

There is no definitive test to diagnose LTBI, which is a clinical diagnosis. The diagnosis of LTBI is done based on a history of prior TB infection and ruling out active TB disease.

PPD Skin Test/ Tuberculosis Skin Test

Purified protein derivative test (PPD skin test), administered through the Mantoux technique, is a type IV hypersensitivity skin reaction to ‘tuberculin.’ Therefore, also known as the tuberculin skin test (TST skin test) and Mantoux test. This test was developed by Koch and further developed by Charles Mantoux, who described the intradermal technique in 1912. 

Tuberculin protein used in the test is extracted from mycobacterium tuberculosis cultures and is used as a purified protein derivative. However, a standardized PPD-S is used, which is a tuberculous mycobacterium (non-tuberculous Mycobacterium are identified by a letter other than S). The results of this test are interpreted by measuring the hypersensitivity reaction (delayed-type hypersensitivity) to tuberculin purified protein derivative, derived from Mycobacterium tuberculosis. The peak of the induration reaction occurs after 24 hours of the test injection. Induration of the skin at the injection site occurs secondary to cell infiltration. 

It takes about 6 to 8 weeks after exposure to the bacteria for the PPD test to be positive. Two visits are required in this test. First visit to get the test administered, and the second visit to get the reading of the test after 48 to 72 hours of test placement.[5]

This test is performed on the patient's skin and no sample is required.[6]

Administration of PPD Skin Test

The FDA approved PPD tuberculin antigens are available for use for this test in the United States. Outside the United States, some other tuberculin antigens such as RT 23 formulation may be used. The dosage of PPD-S and RT 23 formulation differs. The dosage for PPD-S formulation is 5 units in 0.1 ml. No dose adjustments are required in hepatic and renal failure patients.

According to CDC, this test is performed using the ‘Mantoux technique,’ which is injecting 0.1 mL of a solution containing 5 units of tuberculin purified protein derivative into the inner surface of the forearm through intradermal route. It should be administered two or more than 2 inches from the elbow, wrist, or any other injection site. According to the CDC 2005 guidelines, it can be administered to the back of the shoulder if neither arm can be used. 

This test is done using a 1 cc tuberculin syringe and a tiny needle (less than half-inch), with needle bevel of the syringe facing upward while injecting. The needle should be inserted slowly, at an angle of 5 to 15 degrees, and the bevel of the needle should be visible below the skin. The elevation of the skin (6 to 10 mm diameter) known as ‘wheal’ is formed when the small amount of the PPD solution is injected intradermally, which ensures the correct administration. In case of an unsuccessful attempt to form the wheal, the test must be repeated immediately on the other site at least 5 cm/2 inches away from the initial administration site, and mark the second injection site. 

Documentation of the injection site, date and time of test administration, person placing the test, and product lot number and manufacturer, should be done. The patient should avoid scratching or rubbing the area and should keep the site uncovered and clean. Since it is possible to have allergic reactions with tuberculin, epinephrine (1 mg/mL) should be available to treat them.

Reading of the PPD Skin Test

Type IV hypersensitivity reaction (delayed-type) to the injected tuberculin PPD antigen is seen and measured. The reaction starts at 5 to 6 hours, with a peak effect at 48 to 72 hours, after which it begins to subside. Therefore, the right time to read the test is after 48 to 72 hours of intradermal test placement. Erythema and induration are seen at the injection site. The diameter of induration is measured perpendicular to the long axis of the forearm, demarcated, and recorded in mm. Erythema or redness has no diagnostic value and should be ignored. Induration is the palpable, raised swelling, which is measured transversely by inspection and palpation.

It is a time-sensitive test. Tests that are read late are not accurate as they tend to under-estimate the size of the skin reaction. Therefore, the reliability of the test is compromised, and the results are doubtful. To avoid this, repeat testing is recommended if the reaction is not read on time. The second test can be administered as soon as possible. However, if repeated, the test should preferably be performed within 7 days of the initial test to avoid boosting effect. Also, the site of the second test should be on different locations of the body, such as the other arm. 

Proper administration and careful reading of the PPD skin test requires standardization of this test procedure, staff training, and supervision. Although the PPD skin test is commonly used worldwide, its interpretation is challenging.[3]

Tuberculosis Screening Includes:

• Baseline Individual TB risk assessment
• TB symptom evaluation
• TB Screening tests (TB skin test or TB blood test)
• Additional TB evaluation, if required.

The screening tests are not recommended routinely. The screening test should be done in the high-risk population with the intention of treating the latent or active disease if detected. According to 2019 CDC recommendations for TB screening for healthcare personnel, individual baseline TB risk assessment should be done, with no routine annual screening unless any occupational risk or exposure.

According to 2017 guidelines for active or latent TB diagnosis, published by CDC, ATS, and IDSA, indications for TB screening include:

• New infection risk due to exposure to TB

-Employees and residents of homeless shelters and correctional facilities, and some healthcare workers (pulmonologists, respiratory therapists) require serial testing, given their high risk of exposure.

-Close and casual (less than 4 hours/week contact) contacts of untreated, active TB require a single test

• LTBI individuals with increased risk of disease progression. 

-High risk of reactivation (six times higher risk compared to normal). Severe immunocompromised conditions (malignancy, chemotherapy, organ transplant, HIV infection, immunosuppressive therapies) and evidence of healed TB on chest radiograph.

-Moderate risk of reactivation (3 to 6 times risk compared to normal)-Individuals with diabetes and corticosteroid therapy with a high prevalence of TB, i.e., homeless, iv drug users, immigrants from high prevalence areas, active TB contacts 

-Slightly increased risk of reactivation (1.5 to 3 times risk compared to normal)- underweight individuals, smokers, and with evidence of small granulomas on chest radiograph.[7]

The two tests used for TB screening are PPD skin test and IGRA. Test selection based on CDC, ATS, and IDSA guidelines.

• PPD skin test is preferred over IGRA for serial testing of individuals who should be tested on a regular basis. The repeat test result of IGRA is difficult to interpret.
• IGRA is preferred over PPD skin tests when there is a low-intermediate risk of progression from latent to active disease, a high likelihood of patients not returning to get the PPD test read, prior BCG vaccination, or prior history of non-tuberculous infection.
• PPD test can be used as an alternative when IGRA is not available or is too expensive.
• Either test, PPD or IGRA can be used if there is a high risk of progression to active disease.
• Dual testing (IGRA and PPD in any order, if the initial test is positive) is done for low-risk individuals where the law requires latent TB infection testing, and not indicated medically.
• A positive PPD is followed by IGRA testing to confirm the presence of the infection. However, IGRA should be done within three days of initial test placement. LTBI treatment is offered to individuals who are positive for both tests.

Post-exposure indications for PPD screening and testing

According to the CDC 2019 Updated guidelines, TB skin test in healthcare personnel with a known exposure:

• Previous negative TB test result- immediate testing and retesting in 8 to 10 weeks post known exposure.
• Documented prior positive skin test- No need to retest. They should rather have a tuberculosis symptom screen. If they have any symptoms, they should be evaluated further for tuberculosis

The positive skin reaction in the PPD test can occur in:

• Active TB infection
• TB exposure in the past (latent TB infection)
• BCG vaccination in the past (live attenuated mycobacterial strain)
• Infection with variety of non-tuberculosis mycobacteria