Each day, I see people arrive for work after they have had breakfast at home. They sometimes arrive with coffee. Within two hours or so they go to morning break and eat a muffin, fruit, or some other snack, often with more coffee. Two hours later they have lunch, and so on. Our days are one long glucose tolerance test.
Yet we require that patients fast eight to fourteen hours before measuring substances commonly found in all these foods and drinks. The Quest website puts the reasoning succinctly: “If you don’t fast, or fast for a shorter time than prescribed, your tests could give inaccurate results, meaning you’ll likely have to repeat the test.” In the context of lifestyle related testing, “inaccurate” means your results will not compare accurately to other fasting patients, not in the sense of reflecting what’s usually happening in your blood.
“Fasting” is useful for clinicians to make a diagnosis, but it still seems odd to me to ask patients to fast. And in fact, there are guidelines for a random plasma glucose (greater than 200 mg/dL); US News and World Report reported last November that fasting may be unnecessary for cholesterol testing, too. In this study, fasting and non-fasting values varied little, only suggesting a need to repeat triglyceride testing in some instances.
Fasting testing might be unnecessary in some cases. It may have to be, since we never really know how compliant patients are. If many people in work settings don’t last more than a few hours without another sugar, fat, or caffeine load, it’s easy to imagine the torture of a fourteen hour fast. Countless times patients have asked me, “I’m supposed to be fasting, but I just had a ____ and ____,” -- fill in the blanks -- “is that Ok?” I suspect a large number of patients are not fasting. Some probably modify their eating slightly before the test. I’ll bet clinicians know that, too.
Perhaps, cheaper or more reliable testing will allow enough data to be collected to reflect patient habits in a meaningful way. We’ll be fasting a while longer.
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