A “PPT test” most likely refers to one of two common medical tests: the PTT (partial thromboplastin time) blood test, which measures how quickly your blood clots, or the PPD (purified protein derivative) skin test, which screens for tuberculosis. The abbreviations are easy to mix up, so here’s what each one involves and what your results mean.
The PTT Blood Clotting Test
A partial thromboplastin time test, or PTT, measures how many seconds it takes your blood to form a clot. A normal result falls between 25 and 35 seconds. If your blood takes longer than that, it suggests a problem with one or more of the proteins (called clotting factors) responsible for stopping bleeding.
The test itself is a simple blood draw. In the lab, your blood sample is spun to separate the liquid plasma from the blood cells. Technicians then add calcium and activating substances to the plasma to kick-start the clotting process and time how long it takes for a clot to form. The PTT checks nearly every clotting factor in your blood, making it a broad screening tool for bleeding disorders.
Why Your Doctor Orders a PTT
The most common reasons for a PTT test include screening before surgery, investigating unexplained bleeding or bruising, and monitoring patients on the blood thinner heparin. If your result comes back prolonged (longer than 35 seconds), it could point to several conditions:
- Von Willebrand disease: the most common inherited bleeding disorder, where a key protein that helps platelets stick together is missing or defective.
- Hemophilia: a genetic condition where specific clotting factors are absent or produced at very low levels.
- Heparin use: if you’re on this blood thinner, a prolonged PTT is expected and is actually how doctors gauge whether your dose is correct.
A PTT is often ordered alongside a PT (prothrombin time) test. While the PTT evaluates one arm of the clotting system, the PT evaluates a different arm. Together they give a more complete picture of your clotting ability. No special fasting or preparation is typically required before the blood draw.
The PPD Tuberculosis Skin Test
The PPD test, also called the Mantoux tuberculin skin test or TST, determines whether you’ve been infected with tuberculosis (TB) bacteria. It’s a routine screening test required for many jobs in healthcare, education, and food service, and is commonly given during immigration medical exams.
A small amount of a standardized protein solution derived from TB bacteria is injected just under the skin of your forearm. The injection creates a small, pale bump that disappears within minutes. Over the next two to three days, your immune system reacts to the protein if it recognizes it from a previous TB exposure. You must return to a healthcare provider 48 to 72 hours after the injection to have the site read.
Reading Your PPD Results
The provider measures the raised, firm area (called induration) at the injection site, not the redness around it. Whether your result counts as “positive” depends on the size of the induration and your personal risk factors:
- 5 mm or larger is considered positive for people with HIV or severely weakened immune systems.
- 10 mm or larger is the standard threshold for most other people, including children, regardless of whether they received the BCG vaccine (a TB vaccine given in many countries outside the U.S.).
A positive PPD result means your immune system has encountered TB bacteria at some point. It does not necessarily mean you have active TB disease. Most people with a positive skin test have what’s called latent TB infection, meaning the bacteria are present but dormant and not causing symptoms or spreading to others. Further testing, typically a chest X-ray and sometimes a blood test, is needed to determine whether the infection is latent or active.
One important caveat: if you received the BCG vaccine as a child (common in countries across Asia, Africa, Latin America, and parts of Europe), your PPD test can come back positive even without a true TB infection. In these cases, a TB blood test (called an interferon-gamma release assay) is often preferred because it isn’t affected by prior BCG vaccination.
How to Tell Which Test You Need
Context usually makes the answer obvious. If you’re being screened for a new job, enrolling in school, or going through an immigration physical, you’re almost certainly getting the PPD skin test for tuberculosis. If you’re having blood work done before a surgery, experiencing unusual bleeding or bruising, or taking a blood-thinning medication, the PTT clotting test is the one being ordered. When in doubt, the lab order or your provider’s office can clarify which test is scheduled.