TB Skin Tests (Mantoux Test)

A tuberculin skin test (TST) – often called the Mantoux test – is used to detect Mycobacterium tuberculosis infection (latent or active) by intradermal injection of tuberculin PPD. In North America, the standard dose is 0.1 mL containing 5 tuberculin units (TU) of PPD. The PPD solution (Aplisol® or Tubersol®) is injected into the inner forearm with a 27-gauge needle; a small, pale wheal (6–10 mm) should form if done correctly. A properly done test causes no systemic drug effect – it simply establishes whether the person’s immune system recognises TB antigens.

• Administration (Mantoux Method): The patient’s forearm skin is cleaned, and 0.1 mL (5 TU) of PPD is injected intradermally on the volar forearm. The needle bevel is up, and the injection is just under the skin; a visible wheal should appear. If the skin test solution is incorrectly given (wrong volume or too deep), the test is invalid and should be repeated at a new site at least 2″ away. After injection, the patient leaves and returns 48–72 hours later for reading.

• Reading the Test: A trained health professional measures the diameter of induration (firm swelling) at the site – NOT the redness – in millimetres. (If no induration is palpable, record 0 mm.) The reading must be at 48–72 hours; if the patient does not return within 72 hours, the test should be redone (there is no harm in repeating a skin test). Delay beyond 72 hours invalidates the result, as dermal reactions wane.

• Interpreting Results: Interpretation depends on the size of induration and the individual’s risk factors. Current CDC cutoffs are: ≥5 mm is positive in high-risk persons (e.g. HIV infection, close contact of infectious TB, organ transplant recipients, other severely immunocompromised); ≥10 mm is positive in moderate-risk groups (recent immigrants from high-TB areas, injection drug users, residents/workers of high-risk congregate settings like prisons or shelters, certain medical conditions, children <5 years, etc.); ≥15 mm is positive in persons with no known risk factors. Reactivity between these thresholds is considered negative for the corresponding risk group. For example, an induration of 8 mm would be negative for a low-risk adult but positive for someone on immunosuppressive therapy (since it exceeds 5 mm).

• False Positives/Negatives: A TB skin test can yield false results. False positives may occur in persons who received the BCG vaccine or who have nontuberculous mycobacteria – there is no way to distinguish these from true TB infection by PPD alone. Therefore, CDC advises that BCG-vaccinated individuals with positive TST still be evaluated as if they had not been vaccinated. Periodic skin testing can also “boost” old BCG-induced reactivity, leading to persistent positives. By contrast, false negatives can occur in very recent TB infection (it takes 2–8 weeks post-exposure for conversion) or in immunosuppressed states (e.g. AIDS, certain cancers, steroid use). Young infants (<6 months) may also react poorly. Thus, a negative test does not completely rule out infection in high-risk cases; if suspicion is high (e.g. symptomatic patient), further evaluation (e.g. chest X-ray, sputum tests) is done regardless of TST result.

• Two-Step Testing: In some settings (e.g. baseline screening of healthcare workers), a two-step approach is used to avoid misinterpreting a “boosted” response as a new conversion. If the first TST is negative, a second test is given 1–3 weeks later. If the second test is positive, this likely reflects a boosted reaction due to remote infection or BCG, and it is counted as a baseline positive (not a new infection). However, if both are negative, the person is considered TB-free (for now). One-step testing is usually done otherwise.

• Who Should Be Tested: TB skin testing is indicated for people at increased risk of TB infection or disease (household contacts of TB cases, immigrants, healthcare workers, homeless, prisoners, etc.). It is even recommended for all people with HIV infection due to the high risk. By contrast, testing is not needed for people with a documented positive TB test or history of TB disease, since almost all previously infected persons stay TST-positive indefinitely. The only contraindication is a prior severe reaction to PPD (e.g. blistering or necrosis at the site); otherwise, anyone (infant, pregnant woman, person with HIV) can safely receive a TST. (Pregnancy is not a contraindication.)

• Procedure and Handling: The Mantoux TST requires careful technique: use a new sterile syringe/needle for each test, inject intradermally with the bevel up, and confirm a wheal forms. PPD solutions must be refrigerated (2–8 °C) and not frozen. (Refer to product info for specifics.) Reading should be done by a trained provider with a ruler perpendicular to the arm’s long axis.

• Advantages/Disadvantages: The TST’s advantages are that it is simple, low-cost, and well-studied. However, it has drawbacks: it requires two patient visits (administration and reading), skilled administration and interpretation, and a strict cold-chain for reagents. It can cause booster phenomena on repeated testing and is subject to false positives (BCG, nontuberculous mycobacteria) or false negatives in immunocompromise. Rarely, strong local reactions (pain, swelling, ulceration) or systemic allergic reactions (rash, anaphylaxis) can occur. Epinephrine should be immediately available in the unlikely event of anaphylaxis.

• Alternatives: CDC currently recommends newer TB blood tests (IGRAs) as the preferred method in many cases. IGRAs (e.g. QuantiFERON, T-Spot) require one visit and are not affected by BCG vaccination. However, TST remains acceptable when blood tests aren’t available or feasible, or for children under 5 (where IGRAs are less reliable).

Key Points: The Mantoux TST uses a standardised PPD intradermal injection (0.1 mL = 5 TU). The site is read 48–72 hours later by measuring induration. The cutoff for a “positive” test varies (≥5, ≥10, or ≥15 mm) depending on the person’s TB risk. Anyone with a concerning reaction (high induration) requires further TB evaluation (chest X-ray, health assessment). BCG vaccination or non-TB mycobacteria exposure do not exclude a positive result – interpret all results by risk categories. If a person has documented TB infection or a previous positive test, repeating a skin test adds no value. For baseline two-step (e.g. health workers), use a second test after 1–3 weeks if the first is negative. Always follow up any positive TST with a medical evaluation for latent or active TB.

Disclaimer: This information is for educational purposes only. For personal medical advice (e.g. reading your own test, treatment of TB infection), consult a qualified healthcare provider.