Reading TB (Mantoux/PPD) Tests Correctly
Perhaps the most important part of ensuring that results of a Mantoux/PPD tuberculosis test are read correctly is the training of medical assistants and other staff. Some clinics have specifically appointed medical assistants to administer and read PPD tests. These assistants are assigned regular updates and training for current protocol. All healthcare professionals reading such test results should be properly trained, supervised, and updated regularly.
The following equipment should be in every kit for reading TB test results:
- A fine-tipped pen to mark the edges of the induration (palpable, hard, raised area)
- A small, flexible millimeter measuring device/ruler
- A good light source
- The patient's medical record showing the injection site, date, and time
Within 4872 hours after the PPD is injected, the patient should return for a reading. Before the injection is performed, be sure to inform the patient of the date and time the patient will need to return for a reading. If the patient is unable to return within the time frame, adjust the schedule. Be sure to write down the day and time for the site to be evaluated. When the patient returns, check to make sure the time frame of 4872 hours has not expired. If the patient returns beyond 72 hours, the test should be repeated.
Steps for reading the test include:
- Inspect the site
- Palpate the induration (if there is one)
- Mark the induration
- Measure the induration (not the reddened area) in millimeters
- Record the measurement
- Interpret the results (this may be assigned to a physician or other provider who will also recommend/order treatment)
- Treat and educate the patient as appropriate
Inspection requires use of a good light. Look at the site from all angles. Look for the hard, raised induration and ignore the erythema (reddened area) or localized swelling. Sometimes the induration cannot be seen clearly.
Palpation is performed with a light touch of the pads of the fingertips by making sweeping motion over the site. Palpate in a 23 inch area in all directions. This will allow you to isolate the edges of the induration more accurately. Do not push or prod the site.
Mark the edges with a dot at the widest part. You will be measuring from the radial (thumb side) aspect of the induration to the medial (little finger side) aspect, like a watch band lays, not up and down the arm (hand to elbow direction), so mark accordingly.
Measure from the inside of each dot using the millimeter ruler. Record the millimeter measurement along with the date and time it was read, and note any blistering or unusual reactions.
The CDC website has more details and a chart for interpreting the results of a reading, based on the size of the induration and the patient's individual health and social history. It also offers a videotape showing palpation. Go to the CDC website for more information.
Plan to have your next staff meeting focus on proper reading of the Mantoux/PPD tuberculosis skin test. Perhaps you can invite a TB trainer or expert from your local Department of Health (DOH) to be a guest speaker or to lead a workshop. Post the CDC chart on interpretation of TB test results for your clinical staff to reference.
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