From the Guidelines
Yes, you can read a PPD skin test at 47 hours after placement, as it is very close to the recommended 48-hour mark and the reaction will be fully developed. The standard recommendation is to read the test between 48-72 hours after administration, as stated in the guidelines by the American Thoracic Society and the Centers for Disease Control and Prevention 1. When reading a PPD, it is essential to measure the diameter of induration (hardened area) in millimeters perpendicular to the long axis of the forearm, not the erythema (redness).
Key Considerations
- The interpretation of the test results depends on the patient's risk factors, with different cut-off levels recommended for defining a positive tuberculin reaction: ≥5mm for HIV-positive individuals or recent contacts of TB cases, ≥10mm for recent immigrants from high-prevalence countries or injection drug users, and ≥15mm for persons with no risk factors 1.
- If the test is read after 72 hours and is negative, it should be repeated, but positive reactions can still be considered valid even if read late 1.
- The PPD works by causing a delayed-type hypersensitivity reaction in individuals previously exposed to tuberculosis bacteria, with the induration representing the cellular immune response to TB antigens.
Administration and Reading of Tests
- The test should be administered by injecting 0.1 ml of 5-TU PPD intradermally into the volar or dorsal surface of the forearm, using a one-quarter- to one-half-inch, 27-gauge needle and a tuberculin syringe 1.
- The reading should be performed in a good light, with the forearm slightly flexed at the elbow, and the basis of reading is the presence or absence of induration, which may be determined by inspection and by palpation 1.
From the Research
PPD Test Reading Time
- The standard time for reading a purified protein derivative (PPD) test is between 48 to 72 hours after placement 2, 3, 4, 5.
- There is no evidence to suggest that a PPD test can be read at 47 hours after placement, as the recommended reading times are 48 to 72 hours.
- Studies have shown that the amount of induration at 24 hours is highly predictive of eventual findings at 48 to 72 hours, but most decisions should still be based on results measured at 48 to 72 hours 2.
- The size of induration was similar when read at 48 or 72 hours in one study, suggesting that reading the test at 48 hours may be sufficient in some cases 3.
- Another study found that the adjusted mean TST size was larger at 72 hours than at 48 hours, suggesting that reading the test at 72 hours may be more accurate 4.
Variability in Reading Times
- There is significant interobserver variability in reading the PPD test, with a tendency to under-read around the cutoff point 3.
- The use of 5 TU PPD at 48 hours by a trained physician can aid in early and more reliable diagnosis of TB 3.
- The TST should ideally be performed with PPD-S and measured at 72 hours with the ballpoint pen+ruler or palpation+ruler methods 4.