Final answer:
If a PPD test is not read within 48 to 72 hours, it is considered invalid and may not accurately reflect the individual's infection status, potentially leading to false results.
Step-by-step explanation:
If a PPD test for tuberculosis is not read within the recommended 48 to 72 hours, the test is considered invalid. It is crucial for the accuracy of the test that the results are read during this timeframe. If not, the inflammation at the site of the injection might subside or increase irrespective of infection, leading to potential false negatives or false positives. Re-administration of the test is generally advised to ensure reliable results.