A 38-year-old male nurse is evaluated during a routine follow-up visit. Six weeks ago, he … — USMLE Step 3

A $38$ -year-old male nurse is evaluated during a routine follow-up visit. Six weeks ago, he was diagnosed with latent tuberculosis infection after routine occupational screening revealed a positive interferon-gamma release assay. A chest radiograph at that time was normal. He was started on a daily regimen of isoniazid and pyridoxine. Prior to initiating therapy, his baseline hepatic transaminase levels and bilirubin were within normal limits.

Today, he reports that he feels well, exercises three times a week, and has no fatigue, nausea, vomiting, or abdominal pain. He drinks one glass of wine occasionally but has not consumed any alcohol in the past two weeks. Physical examination shows an anicteric patient with a non-tender abdomen and no hepatomegaly.

Current laboratory studies are as follows:

Laboratory Test	Result	Reference Range
Aspartate aminotransferase (AST)	$135$ U/L	$10-40$ U/L
Alanine aminotransferase (ALT)	$165$ U/L	$7-45$ U/L
Alkaline phosphatase	$88$ U/L	$40-120$ U/L
Total bilirubin	$0.8$ mg/dL	$0.1-1.2$ mg/dL

Which of the following is the most appropriate next step in the management of this patient's condition?

Continue the current regimen and repeat laboratory studies in 2 to 4 weeksAnswer
B
Discontinue isoniazid and initiate daily rifampin therapy
C
Temporarily withhold isoniazid and order viral hepatitis serologies
D
Obtain a right upper quadrant abdominal ultrasound
E
Add rifampin to the current regimen to shorten the total duration of therapy

Answer

Continue the current regimen and repeat laboratory studies in 2 to 4 weeks

The correct answer is to continue the current regimen and repeat laboratory studies in

2

4

weeks. Isoniazid (INH) frequently causes a mild, asymptomatic elevation in hepatic transaminases in up to

10\%

20\%

of patients. This phenomenon is often transient, and patients can safely continue therapy. According to clinical guidelines, INH should only be discontinued if transaminases rise to greater than

3

times the upper limit of normal (ULN) in the presence of symptoms (e.g., fatigue, nausea, vomiting, abdominal pain), or greater than

5

times the ULN in the absence of symptoms. This patient is completely asymptomatic, and his AST (

135

U/L) and ALT (

165

U/L) are between

3

and

4

times the ULN. Therefore, he does not meet the criteria for stopping the medication. The safest and most appropriate next step is to continue the current treatment and closely monitor his liver function tests to ensure they do not exceed the

5

times ULN threshold.

Step-by-Step Solution

Analyze the patient's clinical presentation and laboratory results.

The patient is completely asymptomatic but has elevated AST (

135

U/L) and ALT (

165

U/L).

Identifying the presence or absence of symptoms is critical for applying drug toxicity guidelines.

Calculate the degree of transaminase elevation relative to the upper limit of normal (ULN).

AST is approximately

3.4

times the ULN (

135 / 40

). ALT is approximately

3.7

times the ULN (

165 / 45

Guidelines for isoniazid hepatotoxicity rely on exact multiples of the ULN to dictate management.

Apply the guideline criteria for isoniazid-induced hepatotoxicity.

Isoniazid must be discontinued if transaminases are

> 3

times the ULN with symptoms, OR

> 5

times the ULN without symptoms. Because this patient is asymptomatic and his levels are

< 5

times the ULN, he does not meet criteria for stopping the drug.

Up to 20% of patients on isoniazid develop transient, asymptomatic transaminitis that resolves without discontinuing therapy.

Select the appropriate follow-up management.

The patient should continue the medication and have his liver function tests rechecked in

2

4

weeks.

Close monitoring ensures that if the transaminitis worsens to

> 5

times the ULN, the medication can be stopped promptly before fulminant liver failure occurs.

Key Concept

Management of isoniazid-induced hepatotoxicity requires continuing therapy and monitoring if transaminases are less than 5 times the upper limit of normal in asymptomatic patients.

Estimated Time:1m 30s

Question

Answer

Step-by-Step Solution

Key Concept

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