You check, and the meter shows a number. What it means depends on a question the number cannot answer by itself: when did you last eat? The same reading can sit in a very different place depending on timing, which is why readings come with labels. Here are the three you will see most often, why timing matters when you compare numbers, and what published guidance says about each. This is general information from the sources listed at the end, not advice for your situation.
Three kinds of reading
- Fasting. Taken after a period without eating. The lab test is most reliable in the morning after fasting for at least 8 hours, with nothing to eat or drink except sips of water [1]. It is also described as measured after an overnight fast [2].
- Post-meal (also called postprandial). Taken after eating. For home checks, the published US targets are written for 1 to 2 hours after the start of a meal [3].
- Random. Taken at any time, with no fasting needed [2]. A doctor may use it when someone has symptoms of diabetes and does not want to wait until they have fasted for 8 hours [1].
Why timing changes what a number means
Published US targets differ for different moments. For most nonpregnant adults, they list 80 to 130 mg/dL before a meal and less than 180 mg/dL 1 to 2 hours after starting a meal [3]. The after-meal ceiling is higher than the before-meal range, which is why a reading is easier to read alongside when it was taken. Targets are also individualized, based on factors such as how long someone has had diabetes and their age, and more or less stringent goals may suit different people [3]. Your own targets are for you and your care team to set.
What the guidance says about each
These are the published ranges for lab tests used to diagnose diabetes and prediabetes in people who are not pregnant. They are not the same as the day-to-day targets above.
- Fasting plasma glucose: 99 mg/dL or below is normal, 100 to 125 mg/dL is in the prediabetes range, and 126 mg/dL or above is in the diabetes range [1] [2].
- Oral glucose tolerance test: a fasting sample, then a sugary drink, then another sample 2 hours later. At 2 hours, 140 to 199 mg/dL is in the prediabetes range and 200 mg/dL or above is in the diabetes range [1] [2].
- Random plasma glucose: 200 mg/dL or above, together with symptoms of diabetes, is in the diabetes range [1].
Two cautions from the same sources. A blood glucose meter you can buy over the counter cannot diagnose diabetes, and a doctor will usually use a second test to confirm a diagnosis [1]. And if you had a test at a health fair or pharmacy, the doctor may want to re-test you to make sure the result is accurate [2].
Giving each reading its context
Logging your results is described as vital, with results noted alongside factors such as food, activity and stress [3]. Noting whether a reading was fasting, before a meal, after a meal or at a random moment is the same idea: it lets a number be read the way it was measured.
Sources
- National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK), Diabetes Tests & Diagnosis.
- Centers for Disease Control and Prevention, Diabetes Testing.
- American Diabetes Association, Check Your Blood Glucose.
We made Steadion, an iPhone tracker for people with diabetes or prediabetes. In it you can log a fingerstick reading by hand as fasting, post-meal or random, and it saves to Apple Health. Learn more at steadion.fit.