Blood Sugar 88 mg/dl (4.9 mmol/L) — Is That Normal?

Fasting 88, but your doctor still wants another test

It can feel odd to hear “your fasting sugar is normal, but let’s do an A1C” or “let’s do a glucose tolerance test.” If 88 mg/dl is normal, why look further? The reason is that a fasting test answers only one part of the question. It looks at how your body holds glucose overnight. It says much less about how your body copes with the glucose that arrives with a meal.

When several risk factors are present, doctors know that problems can show up in that second area first. A normal fasting value does not rule out prediabetes.

Two different jobs, two different tests

Glucose control has two halves:

• The overnight job is handled mostly by the liver, which releases glucose at a steady pace while low background insulin keeps it in check. The fasting test measures how well this works.
• The after-meal job is handled mostly by muscle, which must take up a large load of glucose quickly, with the help of a burst of insulin. The 75 g oral glucose tolerance test, or OGTT, puts this job under strain on purpose.

Some people have a healthy overnight job but a struggling after-meal job. Their fasting value is normal, yet two hours after the glucose drink they land between 140 and 199, which is impaired glucose tolerance. Without the OGTT that would go unseen. The A1C, which reflects the whole day, can catch some of these people too, sitting between 5.7 and 6.4 % while the fasting value stays in the 80s.

Who is most likely to be offered a second test

Doctors tend to look beyond a normal fasting value when risk factors stack up, for example:

• overweight, especially weight carried around the waist
• a parent or sibling with type 2 diabetes
• gestational diabetes in a past pregnancy, or a baby with a high birth weight
• polycystic ovary syndrome
• high blood pressure, low HDL cholesterol or high triglycerides
• little regular physical activity
• belonging to a group with a higher rate of type 2 diabetes, such as people of African American, Hispanic/Latino, Native American, Asian American or Pacific Islander background

Age matters too. The more of these apply, the more useful a second view becomes.

Preparing for the A1C or the glucose drink

An A1C needs no preparation and can be drawn at any time of day. An OGTT is more involved. You arrive fasting, give a first blood sample, drink a sweet solution with 75 grams of glucose, and give another sample two hours later. You stay seated in between, because walking around changes the result. Eat your usual amount of carbohydrate in the days before; cutting carbs sharply beforehand can make the result look worse than it really is.

Either way, the answer adds detail your 88 cannot give. The prediabetes page explains what each result range means if a second test comes back above normal.