Proper documentation of a tuberculin skin test (TST) requires recording the exact millimeters of induration measured at the injection site, not simply writing “positive” or “negative.” This distinction matters because the threshold for a positive result shifts depending on a person’s risk factors, and a record that says only “positive” leaves future clinicians unable to reinterpret the result when circumstances change. Beyond the measurement itself, thorough documentation includes details about the test administration, the reading window, and the patient’s medical history that could affect interpretation.
What a Complete TST Record Should Include
A well-documented TB skin test result captures two phases: what happened when the test was placed, and what was found when it was read. At placement, the record should note the date and time of injection, the site used (typically the inner surface of the forearm), which arm was used, the manufacturer and lot number of the tuberculin product (purified protein derivative, or PPD), the amount injected (the standard dose is 0.1 mL of 5 tuberculin units), and the name of the person who administered it. At reading, the record needs the date and time the result was measured, the name of the reader, and the induration measurement in millimeters. If no induration is felt, the record should say “0 mm,” not “negative.” This level of detail exists because TB testing often happens in series over months or years, and every reading becomes part of a longitudinal picture. A vague note from a prior test can force an unnecessary repeat or, worse, a missed diagnosis.
Recording Millimeters, Not Interpretations
The single most common documentation error is writing “positive” or “negative” instead of the actual measurement. A 12 mm induration is positive for a recently arrived immigrant from a high-prevalence country but would be interpreted differently in other clinical contexts. If the record only says “positive,” a future provider has no way to know whether the induration was 6 mm or 22 mm, and the clinical weight of those two numbers is very different. Recording the raw measurement in millimeters preserves the information so it can be reinterpreted against any applicable threshold at any point in the future.
This matters especially because the thresholds for calling a TST positive are not one-size-fits-all. They are stratified by risk. For people living with HIV, organ transplant recipients, and others with significantly suppressed immune systems, the cutoff is typically 5 mm or greater. That lower threshold exists because immunosuppression blunts the skin’s ability to mount the inflammatory response the test relies on, so even a small reaction can be meaningful.1Clinical Infectious Diseases. Tuberculin Skin Testing in Patients with HIV Infection: Limited Benefit of Reduced Cutoff Values For other groups with elevated risk, such as healthcare workers, people born in countries with high TB rates, residents of congregate settings, and people with certain medical conditions, the cutoff is 10 mm. For low-risk individuals being tested for administrative or screening purposes, a 15 mm threshold sometimes applies. A measurement of 11 mm could be “positive” under one framework and “negative” under another, which is exactly why the number itself, not the label, belongs in the chart.
How to Measure the Reaction Correctly
The TST measures a delayed-type hypersensitivity reaction in the skin. After injection of tuberculin, sensitized immune cells migrate to the site and cause a firm, raised area called induration. The measurement that matters is the diameter of this induration, measured transversely across the forearm (perpendicular to the long axis of the arm). Redness around the injection site, sometimes called erythema, is not part of the measurement for standard TST and should not be included. You can have a large red area with minimal or no induration, and that is recorded as 0 mm. Confusing redness with induration is one of the more frequent errors among less experienced readers, and it tends to inflate results.
Two techniques are commonly used to identify the edges of the induration. The palpation method involves running a fingertip lightly across the skin to feel where the firm, raised area begins and ends, then measuring that span with a ruler. The ballpoint pen method involves drawing a pen line from the outside toward the center of the reaction from each side; the pen will stop or slow at the edge of the induration, and you measure between those two points. A study comparing the two found no significant difference in their measurements.2PubMed. Palpation vs pen method for the measurement of skin tuberculin reaction (Mantoux test) A separate study looking at how consistently different readers got the same result found that the ballpoint pen technique reduced the zone of imprecision by about 38% compared with palpation alone.3PubMed. Reliability of tuberculin skin test measurement Either method is acceptable, but the pen technique may be worth considering in settings where multiple readers are interpreting tests, since it tends to produce more consistent results across different people.
The 48 to 72 Hour Reading Window
The TST must be read within a specific window after placement. Major guidelines recommend reading the test between 48 and 72 hours after injection, and the World Health Organization specifies that readings should not be taken before 48 hours or after 72 hours.4WHO TB Knowledge Sharing Platform. Annex 2. Tuberculin skin testing: administration, reading and interpretation A test read too early may underestimate the reaction because the immune response has not fully developed. A test read too late may catch a reaction that is already fading, though the evidence suggests that indurations tend to grow rather than shrink over the first few days.
A study comparing readings taken at 48 hours versus 72 hours found excellent overall agreement between the two time points. The induration measurements were clinically different in only about 4% of cases. Where differences existed, the induration was larger at 72 hours in the vast majority of instances, and in 16 cases the result flipped from negative at 48 hours to positive at 72 hours.5PubMed Central. Tuberculin Test Measurement at 48 and 72 hours: mismatch and clinical significance This means that if a patient cannot return at exactly 48 hours, reading closer to 72 hours is generally the better option. A reading taken at, say, 44 hours risks an undercount. If a patient misses the 72-hour window entirely, most protocols consider the test invalid and require replacement rather than a late reading.
From a documentation standpoint, the time of both placement and reading should be recorded precisely enough that anyone reviewing the chart can confirm the reading fell within the acceptable window. Writing “placed Monday, read Wednesday” is adequate if the times of day are close, but noting the actual clock times removes any ambiguity.
Why Erythema Gets Noted But Not Measured
The standard TST is interpreted by measuring induration alone. Redness, blistering, or other skin reactions at the site can be noted in the record as descriptive findings, but they do not factor into the millimeter measurement that determines positive or negative status. This is a point where documentation sometimes goes wrong: a healthcare worker sees a dramatic red flare and documents a large reaction when the actual induration underneath is small or absent.
Research on a newer skin test antigen (a recombinant fusion protein called ESAT6-CFP10, distinct from standard PPD) has explored whether erythema diameter could serve as a useful measurement in certain contexts, finding that erythema improved sensitivity in confirmed TB patients without severely compromising specificity.6PubMed Central. Induration or erythema diameter not less than 5 mm as results of recombinant fusion protein ESAT6-CFP10 skin test for detecting M. tuberculosis infection But that work applies to a different test product, not to the standard PPD-based Mantoux test used in routine screening. For standard TST documentation, erythema is descriptive context, not a measured outcome.
BCG Vaccination History and Its Impact on Records
If your patient received the Bacille Calmette-Guérin (BCG) vaccine, that history needs to appear in the documentation alongside the TST result. BCG is a live vaccine against TB given routinely in many countries but not in the United States, and it can cause the immune system to react to tuberculin even without actual TB infection. A meta-analysis found that BCG-vaccinated individuals were roughly twice as likely to have a positive skin test compared with unvaccinated individuals, though the effect diminished after 15 years for those vaccinated as infants.7PubMed Central. A meta-analysis of the effect of Bacille Calmette Guérin vaccination on tuberculin skin test measurements The same analysis noted that indurations larger than 15 mm were more likely to reflect genuine TB infection than BCG effect.
A common clinical assumption has been that BCG’s influence on TST fades within about ten years, but a long-term follow-up study tracking individuals for up to 55 years after vaccination found that BCG given after infancy could still influence TST results decades later.8PubMed. The Long-term Effect of Bacille Calmette-Guérin Vaccination on Tuberculin Skin Testing: A 55-Year Follow-Up Study This means you cannot safely assume a BCG effect has worn off just because the vaccination was years ago, especially if the person was vaccinated beyond infancy. Documenting BCG vaccination status, including the approximate age at vaccination and whether the patient received one dose or multiple, helps future clinicians interpret a borderline result in proper context.
Cross-reactivity from non-tuberculous mycobacteria (NTM), environmental organisms found in soil and water, can also produce false-positive results.9The International Journal of Tuberculosis and Lung Disease. False-positive tuberculin skin tests: what is the absolute effect of BCG and non-tuberculous mycobacteria? NTM exposure is difficult to document because most people do not know when or whether they have been exposed, but it is worth noting in settings where NTM prevalence is high and TST results fall in borderline ranges.
Two-Step Testing and Baseline Documentation
People entering occupations or settings that require serial TB testing over time, healthcare workers, correctional staff, shelter employees, and others in congregate environments, often need a two-step TST at baseline. The reason involves a phenomenon called boosting. If someone was infected with TB or vaccinated with BCG long ago, their immune memory may have faded enough that the first TST produces a falsely small reaction. The tuberculin injection itself then “reminds” the immune system, and a second test given one to three weeks later produces a larger, more accurate result. Without the two-step baseline, a future single test could show a jump in induration that looks like a new infection (a conversion) when it is actually just the boosted recall of old immunity.10Elsevier / International Journal of Mycobacteriology. Safety of the two-step tuberculin skin test in Indian health care workers
Documenting the two-step process means recording both tests separately with their own dates, times, sites, measurements, and the interval between them. The chart should clearly label which was the first step and which was the second. If the first step is negative and the second step is also negative, the person’s baseline is negative, and any future positive result represents a genuine conversion worth investigating. If the first step is negative but the second step is positive, the interpretation is that the booster effect was detected, the person was previously sensitized, and this is their true baseline. Mixing up the labeling or failing to record that a two-step was performed creates confusion later when serial results are compared.
When TST and Blood Test Results Disagree
Interferon-gamma release assays (IGRAs) are blood tests that serve as alternatives to the TST for detecting TB infection. They work differently: instead of injecting antigen under the skin, they mix a blood sample with TB-specific proteins in a lab and measure the immune response. Because IGRAs use antigens not found in BCG or most non-tuberculous mycobacteria, they are less prone to the false-positive issues that affect the TST in BCG-vaccinated populations.
In practice, TST and IGRA results sometimes disagree, especially in people recently exposed to someone with active TB. A study of household contacts of pulmonary TB patients in Brazil found meaningful discordance between the two tests and suggested that using both may be necessary to catch new infections in recently exposed individuals.11PubMed Central. Discordance of Tuberculin Skin Test and Interferon Gamma Release Assay in Recently Exposed Household Contacts of Pulmonary TB Cases in Brazil When both a TST and an IGRA have been performed, documenting both results together, with their respective dates and the specific IGRA product used, gives the most complete picture. A note explaining the discordance, if present, prevents future providers from assuming one test supersedes the other without context.
This is particularly relevant for BCG-vaccinated individuals who show a positive TST but a negative IGRA. In many clinical settings, the IGRA result is given more weight in that scenario because it is less likely to be confounded by prior vaccination. But neither test is perfect, and documenting both allows future providers to make an informed judgment rather than working from an incomplete record.
Documenting Negative, Anergic, and Unreadable Results
A negative result is still a result, and it deserves the same level of documentation as a positive one. The record should say “0 mm induration” rather than simply “negative” or “nonreactive.” If the patient has risk factors for a false-negative result, those should be documented as well. Conditions that suppress the immune response, including HIV infection, active immunosuppressive therapy, severe malnutrition, and certain cancers, can blunt the skin’s reaction to tuberculin even in the presence of genuine TB infection. This is sometimes called anergy, and it means a negative TST cannot be trusted to rule out infection in someone with a compromised immune system.
Unreadable or invalid tests also need their own documentation. A test is typically considered unreadable if the initial injection did not raise a proper wheal (the small blister-like bump that forms immediately upon injection), if the patient scratched or otherwise disturbed the site, or if the reading fell outside the 48-to-72-hour window. In any of these cases, the record should note why the test was invalid and that it needs to be repeated, rather than recording it as negative.
Practical Formatting for the Medical Record
Whether you are documenting on paper or in an electronic health record, organizing the information consistently helps. A useful documentation template covers these elements in order:
- Placement date and time: when the tuberculin was injected
- Injection site: which forearm, volar surface
- Product and lot: manufacturer name, lot number, expiration date
- Dose: volume and tuberculin units (standard is 0.1 mL of 5 TU PPD)
- Placed by: name and credentials of the person who administered the test
- Reading date and time: when the result was measured
- Induration: transverse diameter in millimeters, or 0 mm if none
- Read by: name and credentials of the person who measured the result
- Interpretation: positive or negative, along with the threshold applied and the clinical rationale for that threshold
- Relevant history: BCG vaccination status, immunosuppressive conditions, prior TST results, and any concurrent IGRA results
The interpretation line is where you apply clinical judgment and document which threshold you used and why. A 7 mm induration in a healthcare worker with a recent known exposure gets interpreted differently from a 7 mm induration in a low-risk office worker getting a routine pre-employment screen. Spelling out your reasoning in the chart protects both the patient and you.
The Purified Protein Derivative Behind the Test
The substance injected in a TST is purified protein derivative, or PPD, a standardized extract of proteins from the tuberculosis bacterium. Its development traces back to Robert Koch’s “Old Tuberculin,” an early and crude preparation that proved unreliable as a treatment but showed promise as a diagnostic tool. Florence Seibert later identified and purified the active protein components, creating the PPD that remains the basis of the modern skin test.12PubMed Central. Latent tuberculosis testing through the ages: the search for a sleeping killer Knowing this is less about day-to-day documentation and more about understanding a quirk of the test: PPD contains proteins shared across several mycobacterial species, which is why BCG vaccination and NTM exposure can trigger false positives. Newer skin test antigens, like the ESAT6-CFP10 product used in some countries, contain proteins specific to the TB bacterium and avoid much of this cross-reactivity. If your facility uses one of these newer products rather than standard PPD, the documentation should specify which antigen was used, because the interpretation framework differs.