If there are pinpoint red spots or slight swelling at the test site, it is considered as a plus-minus sign, indicating further detailed examination is needed. If the erythema and induration at the test site have a diameter of less than 0.9cm, the test is positive. If the diameter is between 0.9 and 1.0cm, it is considered as two plus signs. If the erythema and induration at the test site are greater than 2cm, it is three plus signs. If there are blisters and necrosis at the test site, it is four plus signs, indicating a strong positive result.
PPD skin test, also known as tuberculin skin test, is mainly used to determine whether there is tuberculosis infection in the human body through intradermal injection. For the results, if there is no erythema or induration at the injection site, the test is negative, indicating non-tuberculous infection. For patients who have undergone PPD skin test, the interpretation of the results is usually performed between 48 and 96 hours, with 72 hours being more common. The average diameter of the induration at the test site is measured on the arm where the PPD skin test was performed to interpret the results. If the average diameter of the induration is less than 5mm or there is no reaction, it is considered negative; if the average diameter of the induration is greater than or equal to 5mm, it is considered positive. For patients with positive test results, further classification can be made based on the average diameter of the induration, including general positivity, moderate positivity, and strong positivity. The disappearance time of erythema after PPD skin test is related to the strength of the reaction. Generally, negative results may disappear in about 1 to 3 days, while strong positive results may take 2 to 3 weeks to disappear. Weak positive results fall between the two extremes. PPD test is a tuberculin test, which involves injecting tuberculin intradermally in the forearm. Its main purpose is to determine whether there is tuberculosis infection in the body for the diagnosis of tuberculosis. If the PPD test is negative, the erythema on the forearm skin will disappear within 1 to 3 days. If the PPD test is weakly positive or positive, the erythema on the forearm skin will disappear within about 1 week. If the PPD test is strongly positive, the erythema on the forearm skin may be larger and may be accompanied by blisters, necrosis, etc., which may take 2 to 3 weeks to recover.
