Tuberculin Skin Test (TST) Interpretation Guidelines

Test Administration and Reading

The TST is administered by intradermal injection of 0.1 mL containing 5 TU of purified protein derivative (PPD), producing a discrete wheal 6–10 mm in diameter on the volar or dorsal forearm. 1
The test must be read between 48 hours and 72 hours after placement; reading after 72 hours underestimates induration size and should be avoided. 2
Only the transverse diameter of palpable induration (not erythema) is measured, perpendicular to the forearm’s long axis, in good lighting with the forearm slightly flexed; results are recorded in millimetres, with “0 mm” indicating no induration. The ball‑point pen method reduces inter‑observer variability, and trained personnel must perform the reading. 1

Risk‑Stratified Interpretation Cutoffs

An induration ≥ 5 mm is considered positive in persons at highest risk for tuberculosis, including HIV‑infected individuals, recent close contacts of active TB cases, those with fibrotic chest‑radiograph changes suggestive of prior TB, organ‑transplant recipients or patients on ≥15 mg/day prednisone for ≥1 month, individuals receiving TNF‑blocking agents, and children younger than 4 years or those exposed to high‑risk adults. 2
An induration ≥ 10 mm is considered positive in moderate‑risk groups such as recent immigrants (≤5 years) from high‑TB‑prevalence countries, injection‑drug users, residents or employees of high‑risk congregate settings (prisons, nursing homes, hospitals, homeless shelters, etc.), mycobacteriology laboratory staff, and persons with medical conditions that increase TB risk (silicosis, diabetes, chronic renal failure, hematologic malignancies, certain solid‑organ cancers, >10 % weight loss, gastrectomy, or jejuno‑ileal bypass). 3
An induration ≥ 15 mm is considered positive in individuals with no identified TB risk factors; routine testing of low‑risk populations is not recommended. 3

Test Conversion and Follow‑Up

A rise in induration of ≥ 10 mm within a two‑year interval after an initially negative TST defines a skin‑test conversion, indicating recent Mycobacterium tuberculosis infection. 3

Influence of BCG Vaccination

Prior Bacillus Calmette‑Guérin (BCG) vaccination does not preclude TST use; in BCG‑vaccinated persons who have TB risk factors, a positive reaction should be interpreted as M. tuberculosis infection. Specificity of the TST is reduced in individuals vaccinated after infancy or who have received multiple BCG doses. 2

False‑Negative Results and Interfering Factors

The TST has an approximate 25 % false‑negative rate during the initial evaluation of patients with active TB. 1
False‑negative results are more common in infants and young children, during early infection (< 6–8 weeks after exposure), in persons with overwhelming illness or disseminated TB, after recent viral or bacterial infections, and following recent administration of live‑attenuated viral vaccines (e.g., measles, mumps, rubella, varicella, yellow fever). [2][1]
To avoid vaccine‑related false‑negatives, the TST should be performed either on the same day as a live‑attenuated viral vaccine or 4–6 weeks after vaccination. 1

Limitations of Alternative Testing Methods

Multiple‑puncture skin tests such as the Tine or Heaf methods are insufficiently accurate and should not be used for TB screening. 3

Tuberculin Skin Test Interpretation Guidelines

Introduction to Tuberculin Skin Testing

The American Thoracic Society recommends measuring only the transverse diameter of induration, not erythema, between 48-72 hours after injection, and interpreting using risk-stratified cutoffs: ≥5 mm for highest-risk patients, ≥10 mm for moderate-risk groups, and ≥15 mm for low-risk individuals 4, 5, 6, 7

Timing of Test Reading

The Centers for Disease Control and Prevention recommends reading the test between 48 and 72 hours after injection when induration reaches maximum size 4, 5, 6, 8
Tests read after 72 hours tend to underestimate the true size of induration and should be avoided 4, 8

Proper Measurement Technique

The American Thoracic Society recommends measuring only induration, not erythema, and recording the result in millimeters 4, 5, 6, 8
The measurement should be performed in good light with the forearm slightly flexed at the elbow, using inspection and palpation to determine the transverse diameter perpendicular to the long axis of the forearm 4, 8
The ball-point pen method can be used to decrease interobserver variability 4
Trained personnel must perform the reading—patient or self-reading is not acceptable 5, 6

Risk-Stratified Interpretation Cutoffs

The Centers for Disease Control and Prevention recommends the following risk-stratified interpretation cutoffs:

Skin Test Conversion

The American Thoracic Society defines skin-test conversion as an increase in induration size of ≥10 mm within a 2-year period, indicating recent infection 7

Special Considerations

The Centers for Disease Control and Prevention states that tuberculin skin testing is not contraindicated in persons who received BCG vaccination, and a positive reaction in BCG-vaccinated persons should be interpreted as M. tuberculosis infection when the person is at increased risk for recent infection or has medical conditions that increase risk for disease 7, 9
The PPD skin test has a reported false-negative rate of 25% during initial evaluation of persons with active tuberculosis, and false-negatives can occur due to various factors, including immunosuppression, overwhelming acute illness, and viral infections 4

Common Pitfalls to Avoid

The American Thoracic Society recommends avoiding common pitfalls, including measuring or recording erythema alone, accepting patient self-reading, using multiple puncture tests, interpreting a positive test as "negative" in BCG-vaccinated individuals, and delaying reading beyond 72 hours 4, 5, 6, 7, 8

Mantoux Tuberculin Skin Test: Administration, Reading, and Evidence‑Based Interpretation

Test Administration & Reading Technique

The Mantoux test should be read 48–72 hours after intradermal injection, measuring only the transverse diameter of palpable induration (not erythema) in millimetres; if no induration is present, the result is recorded as 0 mm. 10
Reading must be performed in good lighting with the forearm slightly flexed at the elbow, and the transverse diameter measured perpendicular to the long axis of the forearm. 10
The ball‑point pen method is recommended to reduce inter‑observer variability when measuring induration. 10
Tests read after 72 hours systematically underestimate induration size and therefore should be avoided. 10

False‑Negative Results & Interfering Factors

The tuberculin skin test has an approximate 25 % false‑negative rate during the initial evaluation of patients with active tuberculosis; the rate is higher in immunosuppressed individuals. 10
Live‑attenuated viral vaccines (e.g., measles, mumps, rubella, varicella, yellow fever) can suppress the PPD response, leading to false‑negative results. 10
To avoid vaccine‑related false‑negatives, the TST should be performed either on the same day as the live‑attenuated vaccine or 4–6 weeks after vaccination. 10
Malnutrition, immunosuppressive medications, malignancy, and overwhelming acute illness increase the risk of a false‑negative TST. 10

Common Pitfalls to Avoid (Evidence‑Based Recommendations)

Never measure or record erythema alone; only the induration should be measured and documented. 10
Never use multiple‑puncture tests (e.g., Tine, Heaf) for tuberculin screening, as they are insufficiently accurate. 10
Never delay reading beyond 72 hours, because delayed reading underestimates induration size. 10

Risk‑Stratified Interpretation of the Tuberculin Skin Test (TST)

Interpretation Cutoffs

A positive TST is determined by measuring only the transverse diameter of palpable induration at 48–72 hours; the cutoffs are ≥5 mm for highest‑risk patients, ≥10 mm for moderate‑risk groups, and ≥15 mm for persons with no identified TB risk factors【11】【12】.

Highest‑Risk Groups (≥5 mm)

HIV‑infected individuals (regardless of CD4 count) are considered positive with an induration of ≥5 mm【11】【12】.
Recent close contacts of a person with active TB (exposure ending ≤8–10 weeks ago) are positive with ≥5 mm induration【13】【14】【15】.
Persons with fibrotic changes on chest radiograph consistent with prior TB are positive with ≥5 mm induration【11】【12】.
Organ‑transplant recipients and other immunosuppressed patients (e.g., ≥15 mg/day prednisone for ≥1 month) are positive with ≥5 mm induration【11】【12】.
Patients receiving TNF‑blocking agents are positive with ≥5 mm induration【11】【12】.

Moderate‑Risk Groups (≥10 mm)

Recent immigrants (≤5 years) from high‑TB‑prevalence countries are positive with induration ≥10 mm【11】【12】.
Injection‑drug users are positive with induration ≥10 mm【11】【12】.
Residents and employees of high‑risk congregate settings (prisons, nursing homes, hospitals, homeless shelters) are positive with induration ≥10 mm【11】【12】.

Low‑Risk Groups (≥15 mm)

Individuals with no identified TB risk factors are considered positive only when induration is ≥15 mm; routine testing of this low‑risk population is not recommended【11】【12】.

Contact‑Investigation Protocols

In contact investigations, any contact with induration > 5 mm should receive further diagnostic evaluation, beginning with a chest radiograph【13】【14】【15】.
For children < 5 years who have an initial TST < 5 mm and exposure < 8 weeks, window prophylaxis is advised after excluding active disease, with repeat testing at 8–10 weeks post‑exposure【13】【14】.
The window period for TST conversion is 8–10 weeks after exposure ends; a negative test obtained earlier is unreliable for ruling out infection【14】【15】.

BCG Vaccination Considerations

Prior BCG vaccination does not preclude use of the TST; in BCG‑vaccinated persons who have TB risk factors, a positive reaction should be interpreted as M. tuberculosis infection【11】【12】.
Although specificity is reduced in individuals vaccinated after infancy or who received multiple BCG doses, risk‑stratified cutoffs should still be applied for interpretation【11】【12】.

Factors Contributing to False‑Negative Results

Infants and young children have a higher likelihood of false‑negative TST results【11】.
Patients with overwhelming illness or disseminated TB are more prone to false‑negative results【12】.
Individuals on high‑dose corticosteroids, TNF‑inhibitors, or other immunosuppressive medications exhibit increased false‑negative rates【12】.
Recent viral or bacterial infections can also lead to false‑negative TST outcomes【11】【12】.

Definition of TST Conversion

TST conversion is defined as an increase in induration of ≥10 mm within a 2‑year period after an initially negative test, indicating recent M. tuberculosis infection【15】.
In contact investigations that use the 5‑mm cutoff, any change from negative to positive (≥5 mm) is considered a conversion requiring further evaluation【13】【14】【15】.