MKSAP quiz: Glycemic testing
This month's quiz asks readers about additional testing for a 72-year-old woman with a fasting plasma glucose level of 130 mg/dL (7.2 mmol/L) and an HbA1c level of 5.4%.
A 72-year-old woman is evaluated during a routine wellness examination. She has no concerns and is asymptomatic. Medical history is significant for hypertension. Her only medication is losartan.
On physical examination, blood pressure is 142/92 mm Hg. Other vital signs and physical examination findings are normal. BMI is 29.
Laboratory studies show a fasting plasma glucose level of 130 mg/dL (7.2 mmol/L) and a hemoglobin A1c level of 5.4%.
Which of the following is the most appropriate diagnostic test to perform next?
A. Fasting blood glucose
B. Hemoglobin A1c
C. Oral glucose tolerance
D. Random blood glucose
MKSAP Answer and Critique
The correct answer is A. Fasting blood glucose. This item is available to MKSAP 18 subscribers as item 2 of extension set 2 in the Endocrinology and Metabolism section. More information about MKSAP 18 is available online.
The most appropriate test to perform next is a fasting blood glucose measurement. In the absence of unequivocal hyperglycemia, the diagnosis of diabetes or prediabetes requires two abnormal test results from the same sample or in two separate test samples. The screening tests have different diagnostic sensitivities based on the detected glucose abnormality; thus, discordant results may occur. Additional factors, such as illness or stress, may affect the results of screening tests based on plasma glucose values. This patient has a fasting blood glucose value within the diagnostic range for diabetes with a hemoglobin A1c value in the normal range. When there is a discrepancy in screening test results, the American Diabetes Association recommends repeating the abnormal test. A second fasting blood glucose level of 126 mg/dL (7.0 mmol/L) or higher would be diagnostic of diabetes.
Hemoglobin A1c is an appropriate initial screening test in this patient given that she does not have known factors that could adversely affect the results, such hemoglobin variants, anemia, kidney disease, or liver disease. Repeating the hemoglobin A1c measurement is not warranted as the next diagnostic test in this patient because the initial result was normal.
An oral glucose tolerance test is an appropriate initial screening test for diabetes. Because the oral glucose tolerance test was not among the initial screening tests for this patient, repeating the abnormal fasting blood glucose screening test is a more appropriate next step.
A random blood glucose measurement is useful for the diagnosis of diabetes with a blood glucose level of 200 mg/dL (11.1 mmol/L) or higher in the presence of classic hyperglycemic symptoms, such as polyuria, polydipsia, and polyphagia. However, the patient is asymptomatic with an initial blood glucose less than 200 mg/dL (11.1 mmol/L).
Key Point
- The diagnosis of diabetes or prediabetes requires two abnormal test results from the same sample or in two separate test samples; when there is a discrepancy in screening test results, the abnormal test should be repeated.