How It Works: Professional TB Clearance
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What Is a TB Skin Test?
The TB skin test — also known as the PPD or Mantoux test — shows whether you've ever been exposed to tuberculosis bacteria.
A small dose of purified protein derivative (PPD), a harmless extract of TB proteins, is placed just beneath the skin of your forearm.
If your immune system has encountered TB before, a small raised area forms at the site within 48 to 72 hours.
A trained technician then measures that reaction to read the result as positive or negative.
How the PPD Test Works
The skin test uses the Mantoux method — the standard approach for intradermal TB screening.
Here's how your test unfolds:
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Placement (Day 1): A trained technician injects 0.1 mL of PPD tuberculin solution just below the top layer of skin on your inner forearm. A small raised bubble — a wheal — forms at the site, which is completely normal.
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Waiting period (48–72 hours): Your immune system needs 48 to 72 hours to respond. If you've been exposed to TB before, immune cells gather at the site and create a firm, raised bump known as induration. Leave the area alone during this stretch — no scratching, covering, or applying anything to it.
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Reading (around Day 3): A technician measures the induration — the firm, raised area, not the redness.
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The width of that induration is what determines a positive or negative result under CDC guidelines.
The PPD skin test is recommended by the CDC for TB screening in most healthy adults, and it's accepted by U.S. healthcare employers, schools, immigration authorities, and most occupational health programs.
TB Skin Test FAQ
What happens if I miss the 48–72 hour reading window? The test only counts if it's read inside that window. Past 72 hours the reaction on your arm keeps shifting and can read inaccurately, so a missed reading means starting over — a fresh placement and another two-to-three-day wait. If your timeline is tight, schedule your placement on a day you're certain you can return, or choose the QuantiFERON Gold blood test instead, which is one visit with no return trip.
Can I get the injection site wet or cover it before my reading? You can shower and go about your day normally — water is fine. What you shouldn't do is scratch it, rub it, bandage it, or put lotion or ointment on it, since any of that can irritate the spot and throw off the measurement. Leave it uncovered and untouched until we read it.
What does a positive skin test actually mean — do I have TB? Not necessarily. A positive reaction means your body has been exposed to TB bacteria at some point; it doesn't confirm active, contagious disease on its own. A positive result is typically followed up with a chest X-ray and a provider evaluation to sort out latent exposure from active infection. We'll explain your result and what step comes next.