Test your medicine knowledge with the MKSAP challenge, in partnership with the American College of Physicians.

A 55-year-old man is evaluated following a screening for type 2 diabetes mellitus. He is asymptomatic. He has a history of hypertension and hyperlipidemia. There is no history of anemia, liver disease, or kidney disease. Medications are lisinopril and rosuvastatin.

On physical examination, blood pressure is 123/76 mm Hg and pulse rate is 72/min. His body-mass index (BMI) is 28. The remainder of the examination is unremarkable.

Laboratory studies show hematocrit 45.6%, creatinine 1.0 mg/dL (88.4 µmol/L), fasting glucose 128 mg/dL (7.1 mmol/L), and HbA1c 5.6%.

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

A. Fasting plasma glucose
B. HbA1c
C. Oral glucose tolerance test
D. Random blood glucose

MKSAP Answer and Critique

The correct answer is A. Fasting plasma glucose.

A fasting plasma glucose measurement is the most appropriate diagnostic test for this patient. Diabetes mellitus can be diagnosed with an abnormal result of one screening test performed on two separate occasions. Although the HbA1c is normal in this patient, the fasting plasma glucose is abnormally elevated within the diagnostic range for diabetes mellitus. When discrepant results occur among different screening tests for diabetes, the American Diabetes Association recommends repeating the abnormal screening test. If the repeat fasting plasma glucose measurement is abnormal, the diagnosis of diabetes is confirmed. Screening for type 2 diabetes should begin in all asymptomatic patients at age 45 years. In adult patients with a BMI greater than or equal to 25, screening should occur at any age if one or more additional risk factors for diabetes is present.

Use of the HbA1c as an initial screening test in this patient is appropriate as there is no evidence for anemia or kidney or liver disease that could decrease the reliability of the test. The value was normal and does not warrant a repeat measurement as the next diagnostic test to perform in this scenario.

A 2-hour 75-g oral glucose tolerance test can be used as a screening tool for diagnosing diabetes. Since this test was not initially used for screening in this patient, it is most appropriate to repeat the abnormal screening test (fasting plasma glucose) that was already used for comparison.

A random blood glucose measurement would be useful in this patient if he presented with classic hyperglycemic symptoms in the setting of a blood glucose level of 200 mg/dL (11.1 mmol/L) or above, as that would be diagnostic of diabetes. This patient is not symptomatic.

Key Point

• When discrepant results occur among different screening tests for diabetes mellitus, the American Diabetes Association recommends repeating the abnormal screening test.

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