Wait 48 to 72 hours after the PPD injection
Locate the induration, not the redness
Use a millimeter ruler or calipers
Measure across the forearm, perpendicular to the long axis
Measure only the firm, raised area
Record the size in millimeters
If there is no induration, record 0 mm
Do not measure erythema
Compare the measurement with risk-based interpretation criteria
Consider the result positive, negative, or indeterminate based on the measured induration and risk factors
Have the result read by a trained healthcare professional