February 10, 2008
Diagnosis and Symptoms
Question from Seattle, Washington, USA:
I'm a nursing student and we recently did a laboratory test on metabolism in my physiology class that involved fasting for 12 hours, drinking 75 grams of glucose and then recording our blood sugars every half hour for three hours (basically a glucose tolerance test). My results looked very different than the other students in my class. Time Blood Glucose Fasting 98 mg/dl [5.5 mmol/L] 0.5 hour 179 mg/dl [9.9 mmol/L] 1.0 hour 243 mg/dl [13.5 mmol/L] 1.5 hours 166 mg/dl [9.2 mmol/L] 2.0 hours 138 mg/dl [7.7 mmol/L] 2.5 hours 212 mg/dl [11.8 mmol/L] 3 hours 198 mg/dl [11.0 mmol/L] My professor said he thought I had an insulin secretion problem because my numbers bounced up and down. No one else in the class got a reading over 140 mg/dl [7.8 mmol/L]. Should I follow up with a doctor? I'm 20 years old, thin, with normal blood pressure and lipids so I don't fit the typical profile for type 2 diabetes. What do you think this is?
There are several problems with an exercise such as this. First, the glucose measurements were probably done by fingerstick capillary glucose determinations. You should know that the gold standard results require measurement of glucose from a venous blood sample drawn up and sent to the clinical laboratory There is much greater precision in this than with capillary blood glucose determinations. FDA regulations allow much greater variability in their degree of error. Second, the critical values for assignment of diagnosis are the fasting and two hour post-load glucose values. For instance, your fasting glucose of 98 mg/dl [5.5 mmol/L] is normal. The two hour glucose was 138 mg/dl [7.7 mmol/L] and should be under 140 mg/dl [7.8 mmol/L]. Although patients may have a rise in glucose between 0 and 120 minutes, there are really no cutoff values for assigning diagnosis unless the study is performed as part of an oral glucose tolerance test during pregnancy. My recommendation to you is not to get overly excited about these results Oral glucose tolerance testing is not the usual means to diagnose diabetes and is not being pushed by the American Diabetes Association, although it is a way of making the diagnosis. If your fasting glucose levels are fine, I wouldn’t worry too much. When you get yearly follow-up exams from your primary physician or your gynecologist, have them check a fasting glucose. Only if that is elevated would I get more excited. The reason for glucose levels to be higher than normal may result from inappropriately low insulin secretion or insulin resistance. There is no way to know which is operative unless you do more studies to look at each.