MKSAP Quiz: Diagnosing diabetes

This month's quiz asks readers to evaluate an asymptomatic 37-year-old man whose hemoglobin A1c level is 6.3% and fasting plasma glucose level is 135 mg/dL.

A 37-year-old man is evaluated during a routine physical examination. He is asymptomatic and has no medical conditions.

On physical examination, vital signs are normal. BMI is 28. The remainder of the examination is normal. Hemoglobin A1c is 6.3%, and fasting plasma glucose is 135 mg/dL (7.5 mmol/L).

Which of the following is the most appropriate diagnostic test to perform next?

A. Random plasma glucose
B. Repeat fasting plasma glucose
C. Repeat hemoglobin A1c
D. No additional testing

Reveal the Answer

MKSAP Answer and Critique

The correct answer is B. Repeat fasting plasma glucose. This item is available to MKSAP 19 subscribers as item 4 in Extension Set 2 of the Endocrinology and Metabolism section. More information about MKSAP is online.

A repeat fasting plasma glucose measurement should be performed in this patient to confirm the diagnosis of type 2 diabetes mellitus (Option B). He was appropriately screened for diabetes according to the American Diabetes Association (ADA) and U.S. Preventive Services Task Force (USPSTF) criteria. The ADA recommends screening all adults older than 35 years, and the USPSTF recommends screening adults aged 35 to 70 years who have overweight or obesity. In an asymptomatic individual, diabetes can be diagnosed by two abnormal results based on the following criteria: hemoglobin A1c of 6.5% or greater, fasting plasma glucose of 126 mg/dL (7.0 mmol/L) or greater, or 2-hour plasma glucose level of 200 mg/dL (11.1 mmol/L) or greater during an oral glucose tolerance test. The two abnormal test results can be from the same sample (i.e., a fasting plasma glucose and a hemoglobin A1c drawn at the same time) or two separate samples. If two tests drawn at the same time are discordant, such as in this patient's case, then the test that is higher than the diagnostic threshold should be repeated. The diagnosis of diabetes is made on the basis of the confirmatory screening test. For this reason, fasting plasma glucose should be rechecked for this patient; a repeat fasting plasma glucose of 126 mg/dL (7.0 mmol/L) or greater will confirm a diagnosis of diabetes.

A random plasma glucose or a repeat hemoglobin A1c would not be appropriate for this patient (Option A, Option C). A random plasma glucose should be determined in patients with clear clinical features of diabetes, such as weight loss, polyuria, or polydipsia, none of which are present in this patient. A repeat hemoglobin A1c would be indicated if the initial hemoglobin A1c was higher than the diagnostic threshold (6.5% or greater), which is not the case in this situation. In the event of discordant testing, the abnormal test (the fasting plasma glucose) should be repeated according to the ADA guidelines.

It would be inappropriate to perform no additional testing because this patient has a risk factor for type 2 diabetes (elevated BMI) and has two abnormal screening tests (Option D). His hemoglobin A1c is in the prediabetic range (5.7%-6.4%), and his fasting plasma glucose level was abnormal, indicating a diagnosis of diabetes. Additional testing is needed to refute or confirm the diagnosis of type 2 diabetes and to initiate timely treatment.

Key Points

  • Screening tests for diabetes mellitus include hemoglobin A1c, fasting plasma glucose, or 2-hour plasma glucose during an oral glucose tolerance test; two abnormal tests are required to confirm the diagnosis in asymptomatic individuals, and these tests can be drawn from the same or separate samples.
  • If the two abnormal screening tests are discordant, the test that is above the diagnostic threshold should be repeated to confirm the diagnosis of diabetes mellitus.