What Is a PPD Test?
Tuberculosis is a potentially serious infection that usually occurs in the lungs. The bacteria that causes TB is spread through the air from common things like sneezing and coughing. Unfortunately, that means you are more susceptible to TB in the winter months. We’re all traveling more, attending more festive holiday parties and spending more time around people we don’t normally spend time with. A bunch of people in close quarters is sometimes a good breeding ground for infectious diseases.
The good news is there’s a specific test that can tell you whether or not you’ve contracted tuberculosis. A purified protein derivative (PPD) test or a TST (Tuberculin Skin Test, is a simple test your doctors can use to diagnose you with TB). Read more to learn about the PPD test and whether you need to take it.
What Is a PPD Test?
A PPD test is a skin test that can give you clear answers on your TB status. It helps determine if you have been exposed to Tuberculosis. It does not diagnose disease, just that you have been exposed. Think of a PPD Test as being an allergy test. A PPD protein (called Tuberculin) is injected into your skin and left there for 2-3 days. If your body identifies and reacts to it in 2-3 days, it means you must have been exposed to Tuberculosis bacteria. The test cannot be read earlier than 2 days from the time of injection.
What Is the Fastest Way to Get a PPD Test?
A PPD test involves two visits with your doctor – the first visit is to get the PPD (Tuberculin) injected into your skin and the second visit is to get the area of skin checked and examined to see the intensity of your body’s response the tuberculin protein. As you probably guessed, you will have to wait 2-3 days before the response can be measured.
The fastest way to get this testing done is therefore 48 hours. The response to the test can be checked at a maximum of 72 hours, after which you need to wait before the test can be re-done on a different part of the skin.
What should I be careful about When I Get a PPD Test?
First, you always want to make sure that your doctors’ office will be open at 48 hours from the injection time so that the test result can be read. Or before 72 hours. If you miss that window between 48-72 hours, the test result becomes void.
When you have the first test voided, you have to wait at least 7-10 days before the next PPD test can be injected. And this can become an issue if you have a job or a college form that needs filled. So, our recommendation is that you don’t miss getting the test read between 48-72 hours.
Secondly, many doctors’ offices will not read the test if someone else has done it. They may perhaps agree to read it if you have a letter from a licensed practitioner stating that a PPD was injected in that area (right or left forearm) with a date, time specified to the hour; along with Name of Manufacturer, Lot Number of the PPD vial and Expiration Date of the vial from which the PPD was injected. This is to maintain the integrity of the testing results.
Thirdly, many doctors’ offices will read it as positive or negative because traditionally that is how it was meant to be read. However, the science has progressed and now you need to know the size of the reaction in millimeters. At Statcare, PPD Testing is done all week long and our providers always measure and note the millimeters. If you don’t get the reaction size recorded, it can become an issue with Federal and State requirements such as for purposes of Immigration. Also, all subsequent medical decisions for PPD will be based on the size in millimeters and your medical and demographic history.
What Happens When You Get a PPD Test?
To take the test, a doctor will swab the inside of your forearm with an alcohol solution. You’ll then get a shot administered under the top layer of your skin. It should be quick and relatively painless (for a shot). Your doctor will likely give you a bandage for the injection site but be sure to remove this after a few minutes. Allergy to band-aid glue is common and you want to be sure that nothing interferes with your PPD test. After that, you go home and wait for 48-72 hours.
The second part of a PPD test (NOT to be confused with a 2 step PPD test) happens 48 to 72 hours later. You’ll return to your doctor to get the results. The doctor or nurse will examine the injection point, consider your age, country of travel or birth, medical history including prior PPD results and give you your results. If you experience any redness or irritation during those 48 – 72 hours, it may not be directly related to your PPD test. Don’t panic, just wait until you can see your doctor or nurse to get an accurate reading on your reaction.
Reading the Results of a PPD Test – Common Mistakes
When you arrive for the second part of your PPD test, your doctor will look at the skin around the injection. The standard guidelines are to measure the area that looks reddish or pinkish (called Erythema) and record its size in millimeters. Then, the area has to be felt by the examiner’s fingers to detect any raised, thickened and firm skin inside the red or pink area. This hardened skin is called induration and its size also needs to be recorded in millimeters.
Induration is often raised and has an edge. To the untrained eye, this can easily be confused with redness or bruising that is normal with any shot. You can see that it takes a skilled, trained doctor to administer an accurate reading, which is why it’s important to be selective when choosing a doctor for your PPD test. Always choose a medical provider that does this test routinely.
Although retail pharmacies might offer a PPD test, their staff may not have the same kind of nuanced understanding of the test results to ensure the most accurate reading possible. Pharmacists aren’t trained during their schooling to read a PPD test, but it is a regular part of a doctor’s medical school training. TB is too serious of an infection not to choose the most highly skilled medical professional to help you through a PPD test.
Getting a Positive PPD Test – What It Means
There are a few things to keep in mind if you get a positive PPD test. First, tuberculosis progresses in two stages: latent and active. And all latent TB does not always convert into an active TB.
It is good to think of Latent TB as being a sleeper cell of a terrorist bacteria called Mycobacterium Tuberculosis. The tuberculosis bacteria get inside your body and immediately go into hiding, living inside the very cells that are supposed to fight infections – the white cells. The TB bacteria can thus go undetected for years. Tuberculosis bacteria also multiplies ever so slowly, thus evading detection by your bodies’ troops.
This latent TB phase can last days, weeks, months or years. Latent TB means that you have a TB infection, but the bacteria is in not active yet and isn’t causing symptoms. According to the Mayo Clinic, about 2 billion people in the world have latent TB. It also isn’t contagious, but it can turn into active TB at any time, so it’s still important to keep an eye on.
On the other hand, active TB means that the bacteria started dividing and growing inside your tissues. Typically, the TB bacteria need a lot of oxygen, so they prefer living inside lung tissue where fresh oxygen is always available. Active Tb means you are contagious, and you will experience symptoms. Some symptoms of active TB include:
- Coughing that brings up phlegm
- Cough that lasts three or more weeks
- Coughing up blood or blood tinged sputum
- Chest pain, especially while breathing or coughing
- Unintentional weight loss
- Fever, esp. towards the evening
- Night sweats
- Loss of appetite
Unchecked, TB can also affect other parts of your body including kidneys, spine, and brain. It is important to talk to your doctor immediately after a positive PPD test to get a treatment plan in place.
What Does a Positive PPD Test Look Like – Common Mistakes
Indurations that indicate a positive PPD test look different for different kinds of people. Your previous health-related issues, plus your lifestyle can both affect your positive reading. For a healthy person who has a normal immune system, an induration greater than or equal to 10 mm is considered a positive.
A common mistake is to measure it along one dimension. The induration needs to be felt by rolling the fingers all around its edges. Then the maximal edges should be marked with a pen. Finally, the distance between any of the two furthest points has to be measured using a straight metric ruler – one that that has markings in millimeters.
Another mistake that gets made is when the skin is inflamed and the PPD is read as a positive. If blisters are present on that person’s arm, the medical professional needs to determine if it was an allergic reaction to the protein (hypersensitivity) or truly a positive result. It is quite common to have an allergic reaction to the glue in the tape. This glue upon the skin causes an inflammation of the skin (contact dermatitis) which may have nothing to do with the underlying PPD injection. Again, pharmacists are not trained medically to be able to read such test results at It needs a background medical knowledge about these medical conditions and clinical experience for it to be diagnosed properly.
Also, not everyone has a typical immune system. For those, the test gets a little more specific. For example, if an induration measures 4 mm or more, the USCIS considers it a positive test for purposes of the i693 Form that is filled out by a USCIS Certified Civil Surgeon for purposes of Immigration to the US.
An induration of 5 mm can be indicative of TB for people who:
- Have suppressed immune systems
- Are HIV positive
- Have seen changes on chest x-rays that are consistent with previous TB
- Are recipients of organ transplants or on steroids, chemotherapy drugs or TNF-Alpha-blockers
If an induration measures 10 mm or more, it is considered a positive test if that person qualifies as one or more of the following:
- A recent immigrant (within 5 years) from a place with a high prevalence of TB
- A recent travel of more than a month in areas with a high prevalence of TB
- A resident or an employee working in a high-risk area (including hospital and urgent care staff)
- IV drug abusers
- Children under 5 years of age
- People with higher risk of converting from latent TB to active TB – those with kidney disease, diabetes, stomach surgery, certain cancers, Body Mass Index of less than 18.5%, silicosis etc.
An induration of 15 mm or more is considered a positive test if that person qualifies as being low risk and who would otherwise not have needed PPD testing for screening for TB.
As you can see, there is a lot of personal medical history that goes into reading a positive PPD test. If you want to talk to your doctor and ask questions about your PPD test, you should seek out a testing center that you can trust, with doctors who are prepared to give you accurate and complete answers. Statcare doctors can help you through treatment options and offer all the resources you need right at your appointment. No need to follow up or see additional doctors. You’ll get everything you need during your visit.
An important note on positive PPD tests: your induration will likely not go away for 3-4 weeks. That is normal. If it becomes red and starts creating ulcers, talk to your doctors.
What to Do After a Negative PPD Test
If there is no induration present at the site of the test, you most likely do not have TB. However, there are instances such as viral infections, vaccinations and certain disease states and medicines that you might be consuming where you can get a false negative for a PPD test. People who have had their immune system compromised, by chemotherapy treatments, AIDS virus or steroid usage can experience a false negative PPD test. Be sure to answer all your doctor’s questions honestly when giving your medical history so they can determine whether your PPD test is negative or may be presenting a false negative.
What is a 2 Step PPD Test
So far, we discussed a regular PPD Test and how the negative results could be falsely negative. Studies have shown that repeating the PPD Test after a 7 to10 day interval, boosts the response and increases the likelihood of eliminating the false negative test results. In other words, a re-exposure to the tuberculin protein is more likely to enhance the ability of detecting the exposure to TB. This is called a 2 Step PPD Test or a 2 Step Tuberculin Test. It is typically done for all healthcare related job physicals and needs about 2 weeks to complete. This is another reason why getting a regular or a 2 step PPD test done at a trusted medical facility rather than a pharmacy or other location is so important.
Follow Up PPD Tests
Your care does not stop once you are read your PPD test results. If you test positive, there is no need to take the test again. Once you have been exposed to Tuberculosis, that test will always come back positive for the rest of your lifetime. The follow-up conversations you’ll have with your doctor will include medication and symptoms management. TB responds to different medication in a unique way, so getting treatment as soon as possible will help you maintain a healthy lifestyle. You’ll want to schedule 2 chest x-rays, one year apart, to monitor your lung function after your initial diagnosis. Women who are pregnant will also need special care, so keep in touch with your doctor as you manage your TB.
On the other hand, a negative result doesn’t mean that you will remain negative forever. Everyone should consider taking an annual PPD test, but especially if you fall into one of the more vulnerable categories listed above. At all Statcare locations, you can get an accurate PPD test administered and read by a doctor you trust, for only $40.
So, Should You Get a PPD Test?
If you live in NYC, the answer is simple: yes. Exposure to Tuberculosis is common in NYC. If you think you may have been infected, been around someone who has TB or recently traveled to high-risk areas, definitely ask for a PPD test as soon as possible. Come to Statcare for your 2 step PPD test as well as all the answers and treatment options you need. We are here to help.