A 62-year-old white woman, a retired elementary school teacher, presents to your office for a routine tuberculin skin test required for renewal of her teaching license.
Examination
- Patient looks well and is in no distress
- Weight 138 lbs; height 66 inches; body mass index 23; other vitals normal
- Normal heart, lung, and abdominal exam. No cervical or other adenopathy.
Medical history
- Has osteoporosis for which she takes alendronate weekly; also has diet-controlled hyperlipidemia, and is status post-remote hysterectomy for uterine bleeding
- Married, with 1 grown son who is healthy
- Nonsmoker; drinks 2 alcoholic beverages weekly; no illicit drugs
- Walks 40 minutes 4 times a week
- Mother died at age 93 from congestive heart failure; father alive, 92, has bladder cancer
- Review of systems negative for cough, fever, weight loss, or swollen glands
A Mantoux tuberculin skin test (TST) is administered per clinic protocol, and the patient is instructed to return in 48 to 72 hours for a reading of the test result.
The Mantoux TST is the most accurate test for determining tuberculosis (TB) infection.1 The standard procedure uses 0.1 cc (5 tuberculin units) of purified protein derivative (PPD) in a standard tuberculin syringe (3/8 inch, 26–27 gauge). This is administered on a flexor surface of the forearm, 2 to 4 inches below the elbow, and requires an intradermal injection (needle bevel upward) that raises a wheal 6 to 10 mm in diameter. A previously reported positive TST does not contraindicate repeated administration.1
Q: What is a positive TST result? What is the correct technique for reading a TST? Are other tests available to confirm a positive TST?
A:
Interpreting TB test results
The definition of a positive tuberculin skin test result depends on a person’s risk factors as defined in the TABLE.1 Read a TST result 48 to 96 (ideally 72) hours after administration. Palpate and measure induration (not redness).
An alternative method is to use a ballpoint pen to draw a line starting at 1 cm from both sides of the skin reaction and moving toward its center. Where you encounter increased resistance, mark that as the border of induration. Then measure the distance between the 2 borders. This method has been reported to be slightly more precise than palpation.2
Another recently developed test for TB infection, the QuantiFERON, is based on quantification of interferon-gamma response in whole blood to TB infection. Its routine use is not recommended by the Centers for Disease Control and Prevention (CDC).3 It was unavailable in the county where the patient was tested.
TABLE
TST results regarded as positive for tuberculosis, given a patient’s specific risk factors
| 5 MM | INDURATION 10 MM | 15 MM |
|---|---|---|
| HIV infection Close contact of person with TB Previous TB on chest x-ray Intravenous drug use, or unknown HIV status | Native of country with endemic TB HIV-negative intravenous drug user Low income, inadequate healthcare Resident of long-term care facility Medical condition with increased TB risk Ages less than 4 years Likely exposure to TB | Patient has no risk factors |
The patient’s return 48 hours later
A nurse examines the patient’s arm and is uncertain how to interpret the test result. The patient’s primary physician is not in the clinic, and one of the other physicians is consulted. He reads the result as “20 mm induration, positive for TB infection.“
A chest X-ray shows no evidence of tuberculosis infection or other abnormality. The patient is referred to the local county health department. In accordance with CDC guidelines, she is diagnosed with latent tuberculosis infection and started on daily isoniazid therapy.4
The patient’s family physician learns of the patient’s diagnosis after she starts isoniazid therapy. Because of her low risk of tuberculosis, he wonders if the skin test result might have been misread. He discusses the issue with the physician who read the first skin test and discovers there was uncertainty regarding the redness (as opposed to the induration) of the skin reaction. The patient herself reports that she did not feel a hard “bump” on the skin where the test was administered.
The family physician informs the patient that the tuberculin skin test may have been incorrectly read as positive. He gives her the option of repeating the test at the county health department, whose personnel are experienced in administering and reading tuberculin skin tests. The patient chooses to repeat the TST, which is read as definitely negative. Isoniazid therapy is stopped. The patient is grateful that she does not have to continue unnecessary and potentially harmful therapy.