When should a Purified Protein Derivative (PPD) test be read?

From the Guidelines

A PPD test should be read 48 to 72 hours after administration. The healthcare provider will examine the injection site for induration (hardened, raised area), not just redness, and measure its diameter in millimeters. It's crucial to have the test read within this specific timeframe because reading it too early might miss a reaction that hasn't fully developed, while reading it too late could lead to false results as the reaction may have already begun to fade.

Key Considerations

• The test works by detecting if your immune system recognizes tuberculosis proteins, indicating previous exposure to TB bacteria.
• If you miss the 48-72 hour window, you'll likely need to have the test repeated, as readings outside this timeframe are not considered reliable.
• Only trained healthcare professionals should interpret PPD results, as proper measurement technique and knowledge of individual risk factors are essential for accurate interpretation, as stated in 1.

Administration and Interpretation

• The test is administered by injecting 0.1 ml of 5-TU PPD intradermally into the volar or dorsal surface of the forearm, as described in 1.
• The transverse diameter of induration should be measured and recorded in millimeters, with a reaction of 5 mm or greater considered positive for close contacts of tuberculosis cases, immunosuppressed persons, and individuals with clinical or radiographic evidence of current or prior TB, as outlined in 1.

Importance of Timely Reading

• Reading the test within the 48-72 hour window is essential to ensure accurate results, as tests read after 72 hours tend to underestimate the true size of induration, as noted in 1.
• The sensitivity and specificity of the tuberculin skin test are critical in determining the presence of latent tuberculosis infection, and timely reading is essential to ensure that individuals at high risk are evaluated and appropriately treated, as discussed in 1.

From the Research

Timing of PPD Test Reading

The timing of reading a Purified Protein Derivative (PPD) test is crucial for accurate results. According to the studies:

• The optimum time for reading tuberculin tests was found to be in the period 48 to 72h after the injection of bovine PPD 2.
• Indurations were read after 48-72 h in a study evaluating the interaction between C-Tb and PPD 3.
• A prospective study found that the amount of induration at 24 hours was 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 4.
• The appearance of an indurated area of 5 mm or more 48 to 72 hours after administration of purified protein derivative (PPD) is considered a positive reaction in certain individuals 5.
• A comparison of different methods and times for reading the tuberculin skin test found that the adjusted mean TST size was larger at 72 h than at 48 h and 96 h, and the smallest number of false results was observed at 72 h 6.

Methods of Reading PPD Test

The methods of reading a PPD test can also affect the results. The studies found that:

• Subjective interpretation by palpation was more efficient than caliper measurement as a means of detection of tuberculous cattle 2.
• Significant differences were noted between the pen and palpation methods, and results with the pen technique must be interpreted with caution in the 5- to 14-mm range 4.
• The ballpoint pen+ruler or palpation+ruler methods had the least variability at 72 h 6.

Key Findings

Key findings from the studies include:

• The PPD test should be read 48 to 72 hours after injection.
• The ballpoint pen+ruler or palpation+ruler methods are recommended for reading the test.
• The test results should be interpreted based on the individual's risk factors and medical history 3, 2, 4, 5, 6.