With the TB skin test test, the results of the test appear in 48-72 hours and the skin reaction usually lasts for about a week. This skin test involves injecting a pure protein derivative of TB into the skin of the forearm in 0.1 ml and measuring the diameter of the nodule, if the diameter exceeds 10 mm it is considered positive, a positive test suggests a higher risk of developing TB disease, 90% of contacts with a test diameter of more than 10 mm are infected with TB, and if the diameter is more than 15 mm it is 100% infected.The PPD test is only suitable for healthy people The results of the PPD test should not be used to determine exposure in HIV patients with low CD4 counts, patients on immunosuppressive therapy, or patients who are severely malnourished. It is judged positive if the diameter exceeds 5mm in people who are HIV positive, immunocompromised, or have a recent history of household exposure, 10m in people with moderate risk of exposure, and 15mm in people with no risk factors. Patients with a positive PPD test should undergo a chest radiograph or lung CT, and if lung lesions are found, three further sputum specimens for culture or smears are required to further define the diagnosis.