ABIM Exam Review Practice Question of the Week: TB Screening Using the PPD

Here’s a practice question directly from Knowmedge’s Internal Medicine Board (ABIM) Exam QVault.

38-year-old female with history of rheumatoid arthritis and autoimmune hepatitis is applying for a job as a nurse and needs to have a physical examination performed before beginning work. For the past year, the patient has been on prednisone 20mg daily and sulfasalazine for her rheumatoid arthritis. Her arthritis has been well-controlled on the current regimen. As part of the pre-employment process, a purified protein derivative (PPD) test is required. PPD is placed and basic blood work is obtained. She returns two days later and induration size of the PPD is 6mm. Chest xray is negative. CBC is normal except for some mild normocytic anemia because of her rheumatoid arthritis. Comprehensive panel shows mildly elevated LFTs from her autoimmune hepatitis. Which of the following is recommended for this patient’s PPD results?

A. Treatment with rifampin for four months

B. Treatment with isoniazid (INH) and vitamin B6 for nine months

C. No treatment required

D. Treatment with rifampin, isoniazid, pyrazinamide, and ethambutol

E. Repeat PPD now

Explanation

The PPD is a commonly performed test, particularly for pre-employment purposes. Its utility is dependent on the practioner’s ability to risk-stratify the patient. Only after obtaining the patient’s medical history, medication use, residence, and social history can the physician determine what would constitute an abnormal test and necessitate further work-up. Induration size cut-offs are at 5mm, 10mm, and 15mm.

● Induration size of the PPD greater than 5mm is considered positive for patients with HIV, recent TB exposure, old TB scar, long-term prednisone use of more than 15mg/day or those with a prior transplant.

● Induration size greater than 10mm necessitates further work-up for health care workers, homeless individuals, nursing home residents, or alcoholics. Other considerations should be given to patients with lymphoma, silicosis, leukemia, diabetes and chronic renal failure. Additional consideration needs to be given to those going through chemotherapy, as well as patients who are IV drug users.

● All other patients can have an induration size up to 15mm before it is considered abnormal.

This patient is on 20mg of prednisone so she would need treatment if the induration size is greater than 5mm. This patient has induration of 6mm which would require a chest xray, which was performed. The patient’s high induration size and normal chest xray indicate she has latent tuberculosis (TB). This eliminates Choice C (No treatment required) and Choice E (Repeat PPD now). Choice D (Rifampin, INH, PZA, and ethambutol) would be correct if the patient had active TB.

Now we have to choose between Choices A and B. Usually INH and vitamin B6 are given for 9 months but this patient has elevated liver function tests which would be a contraindication for INH.

Choice A (Rifampin for 4 months) is the correct answer. Rifampin will help treat the patient’s latent TB.

You can see all the previous ABIM Exam Review Questions of the Week at the Knowmedge Blog. You can also find additional topics and questions directly from the Knowmedge Internal Medicine ABIM Board Exam Review Questions QVault.