The really short answer for the exceedingly vast majority of patients is an emphatic NO. Tuberculosis (TB) is an airborne bacteria spread by coughing making it a public health threat. There are two tests for tuberculosis, a skin test and a blood test. The skin test is made from a purified protein derivative (PPD) of the tuberculosis bacteria. A +PPD test result could be an indicator of latent or active TB infection. Great, latent or active, so now I have two diseases to worry about? Nope, it is the same disease in different forms. An overly simplified analogy could be shingles. The virus is always in your system, but may only activate under certain circumstances where it can take a foothold and grow, causing a very uncomfortable rash. Sorry if you didn’t know that, but yes you may have another shingles eruption later in life. More importantly, latent TB is NOT contagious. In this phase there is NO tuberculosis bacteria in your saliva/sputum.
Therefore, a +PPD theoretically means you may have either active TB, latent TB, or have been exposed to a portion of the virus. Active TB is what a radiologist is looking for on chest X-ray. Latent TB is in the quiet phase and has traditionally been treated with a six month cocktail of medication, where you cannot drink alcohol and need to be strict and diligent about taking the whole course for six months. Either one of these two factors can pose obvious significant challenges to any patient. These patients will require a chest X-ray to hopefully confirm that they are in a latent and not active phase.
Our last group of +PPD patients are a larger global population and in some countries now older populations with vaccination no longer deemed necessary. These are people who were given the Bacillus Calmette–Guérin (BCG) vaccine in areas where TB is more common. The vaccine was originally developed from a strain of similar bacteria affecting cows, made less effective, but still alive. We get the vaccine which our body recognizes as foreign but doesn’t make us noticeably sick, and our immune system fights back. Brilliantly, the “cow TB” looks so similar to “human TB” that our bodies develop cross reactivity (your body can now fight “cow TB” and more importantly “human TB”) making us immune. So, if you have ever had the BCG vaccine against TB, then you will have a +PPD test the rest of your life. Because of this, some of these people are screened for active TB with a frontal chest X-ray. Basically, if you test positive you should expect a chest X-ray at some point in your life, and likely several scattered throughout the course of your life, depending on your healthcare providers clinical judgement and possible legal requirements.
Finally we get to the Chest X-ray. So what the heck is the radiologist looking for anyways? Looking at the chest X-ray you can see how near a person’s head the lung size is smaller, and blacker like the air in the room. The larger bottom of the lungs has more blood vessels than the top. This sets up a gradient in the lungs with more air near the top, like in a pool or larger body of water how it is all water but near the top or edge it is warmer than at the bottom. TB likes more air, so it tends to occur at the top and will produce holes in the lungs. As with other infections and disease states, your lymph nodes can get big and swell up in the center of your chest, which we can see on X-ray too. There is a blood-borne pattern that can be seen in the lungs historically compared to the size of a millet seed. This appears like thousands of tiny dots on the lungs in the chest X-ray, only seen in textbooks by most current radiologists in the U.S.
The grand takeaway is that if you have a +PPD it is highly unlikely that you have active contagious TB, even if you are coughing. It is statistically much more likely that you have a viral cold or possibly pulmonary infection/pneumonia. That being said, this article stemmed from a discussion between myself and a technologist today. I suggested this as a general rule to her. If the prescription states +PPD and the patient has NO cough or fever/asymptomatic, then I would not be concerned about contracting TB. Hopefully, if the patient is coughing, then they will already have a mask on or have acquired one from the front desk of their healthcare facility. I’m not trying to dissuade anyone from responsible public health behavior , but I would like to alleviate patient and healthcare providers potential anxiety in relation to the presence of a +PPD. Having practiced in New York close to Manhattan with a beautiful, tremendous amount of cultural diversity, and currently seeing on average 2-3 chest X-rays a week with the indication +PPD, I have read a large number of films and still have never seen a millet pattern first-hand/di novo and only several cases with holes/cavitation and scarring at the top of the lungs. It is always wise to practice universal precaution, which is treat everyone as if they are infected and contagious, but in the real world we must also be practical and people should not freak out over the dreaded +PPD.
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