Can you read a tb test after 72 hours?

Are you curious if it’s possible to read a tuberculin skin test (TB test) after the recommended time? Well, look no further because we are going to dive into this topic headfirst. Whether you forgot your scheduled appointment or just missed checking the results in time, let’s explore whether reading a TB test after 72 hours is worthwhile.

Understanding TST Results

Before we go any further, let’s quickly review what happens during a tuberculosis skin test and how the results are interpreted. The tuberculin skin test involves injecting tuberculin under the top layer of your skin on your forearm. If your immune system has encountered tuberculosis bacteria before, it will react by forming an area of redness around the injection site within two to three days.

Unless there is obvious swelling at the injection site beyond that of normal response, less than 5 mm should be considered negative for mantoux testing with purified protein derivative (PPD). However, reaction size must always be evaluated along with risk history factors such as exposure and symptoms suggestive of disease.

Healthcare professionals measure this reaction size using millimeters (mm). A positive result occurs when there is an induration – not simply redness – greater than or equal to:

  • Five mm for people who have HIV
  • Ten mm for people who have traveled from high-prevalence areas outside of North America
  • Fifteen mm for all other individuals aged five years and above without known exacerbating health conditions

Knowing these measurement criteria upfront can lessen anxiety while waiting to hear back about potential exposure to TB through contact tracing evaluations.

What Happens After 48-72 Hours?

You may encounter circumstances where you miss reading your results within their required timeframe. Reasons may include:

  • Forgetting
  • Work scheduling conflicts
  • Life events such as accidents or emergencies
  • Zombie apocalypses (Ok, we hope you do not have to face a zombie apocalypse)

You might be wondering, will the results still be accurate? The short and sweet answer is – no. You cannot read a TB test after 72 hours at all as it becomes invalid at this stage.

Once the TB antigen enters the system through injection in your skin or arm, your immune cells respond with an area of redness and swelling that peaks at around forty-eight to seventy-two hours post-injection. Beyond seventy-two hours, reaction size can change due to natural processes inherent to your body causing false-positive results.

In other words, just because we missed an appointment does not mean we get rewarded for taking extra time. Life may give us lemons sometimes but please try not to fall down before getting up again!

Causes of False-Positive Results

Did you know that several factors can cause a positive result on a tuberculosis skin test even when there is no active infection present in the body? It’s true! Here are some possible explanations:

  • Receiving multiple BCG vaccines
  • Infections by non-tuberculous mycobacteria
  • Certain types of fungal infections
  • Cancer
  • Lupus

If any one or more risk factor seems relevant enough for further screening recommendations from recent exposure history until confirmed negative status checking thereafter routine testing approximately every two years could help alleviate future unnecessary stress.

What Should You Do After Missing Your Appointment?

Just because reading a TB test after 72 hours isn’t recommended doesn’t mean all hope is lost. Give yourself grace! If circumstances prevented you from attending your appointment within the required timeframe schedule another appointment as soon as practically possible so that prompt management occurs if need be afterwards.

Most importantly seek advice from medical professionals regarding proper next steps especially if exposure signs exist upon identifying it early enough regardless minor risk factors else waiting appropriately long intervals prior retesting; whether blood or skin tests can reduce health risk for future infections.


Reading a TB test after 72 hours is not possible, and delaying the reading may do more harm than good. Knowing the requirements for a positive result ahead of time ensures less confusion leading to anxiety waiting out results.

In summary, measuring reaction size in millimeters remains paramount to accurate tuberculosis screening which must be determined within forty-eight to seventy-two hours post-injection as anything beyond could lead to false-positive results on an individual’s medical record until confirmed negative status checking thereafter.

The next time you encounter circumstances preventing scheduled appointments from occurring, try not to panic; take action in seeking appropriate guidance before exposure becomes rampant leading back stressing about abnormal signals that should have been caught sooner rather later by avoiding embarrassment when presenting at work or other engagements down the road with new friends still unaware of interactions around others who suffered due lack urgency whenever it was required.