Sugar in urine but not in blood is not possible — the body will always have blood sugar. But having higher than normal glucose levels in urine could mean diabetes.

Blood sugar, also called glucose, is the body's main source of energy. It's impossible to not have glucose in your blood, because the body needs glucose to survive. Having blood sugar in your urine is another story, though. If there's glucose in your urine, it can be a sign of uncontrolled diabetes, according to the U.S. National Library of Medicine 1.

How Glucose Is Processed

The body gets most of its glucose from metabolizing the carbohydrates in food, according to Kaiser Permanente. The hormone insulin helps move glucose out of the blood and into the cells; through this process, insulin lowers blood sugar levels.

Normal blood sugar levels are as follows, according to the Cleveland Clinic 245:

  • Fasting blood sugar (no food for at least 8 hours): less than 100 mg/dL
  • Random blood sugar (regardless of the time of last meal): less than 140 mg/dL

Blood sugar levels that are higher than these are considered to be outside the normal range, a condition called hyperglycemia. Hyperglycemia is a key concern for people with diabetes, according to the Mayo Clinic 45. This is because their bodies either don't produce enough insulin or can't use the insulin that they do produce effectively.

Improper insulin action means that not enough glucose is moved into the cells. Therefore, it builds up in the bloodstream, causing high blood sugar. If blood sugar levels get too high, glucose may be present in urine.

Read more: What Is a Healthy Blood Sugar Reading in the Morning?

Sugar in Urine

Because the systems of people with diabetes aren't able to process blood sugar normally, their bodies need to reduce blood sugar another way. At first, the kidneys will attempt to filter and absorb the excess glucose, according to the Mayo Clinic 45. But if blood sugar levels are consistently too high, the kidneys won't be able keep up — and the excess glucose will eventually come out in the urine.

When the kidneys have to work to flush out extra sugar in the blood, it can result in frequent urination and dehydration — both of which are common symptoms of diabetes. 

Should You Test Your Urine for Glucose?

You can have your urine glucose levels checked at your doctor's office or do it yourself with an at-home kit, per the Library of Medicine 1. Normal glucose levels in urine should fall between 0 and 15 mg/dL (which is a very small amount), according to the University of California San Francisco 6.

However, Samar Hafida, MD, an endocrinologist at Harvard's Joslin Diabetes Center, tells that testing urine glucose levels is no longer common practice. "We don't really use urine glucose for monitoring purposes in the U.S. [anymore]," she says. "[The test] may be used in places that don't have a lot of resources, but where glucometers [blood sugar-testing devices] are available, we don't use urine glucose for anything. 1"

Dr. Hafida explains that blood glucose tests are much more accurate than urine glucose tests when it comes to monitoring diabetes. Urine glucose tests can be inaccurate, she says, and are likely to have confusing or simply unhelpful results.

Another thing that makes them impractical is that diabetes medications called SGLT-2 inhibitors actually cause glucose to be excreted in urine. These medications — which include empagliflozin (Jardiance), canagliflozin (Invokana) and dapagliflozin (Farxiga) — lower blood sugar by preventing the kidneys from absorbing too much glucose in the first place. The excess glucose is flushed out through urination instead.

If you have glucose in your urine but a normal blood sugar reading and you're not taking an SGLT-2 inhibitor, the blood test or glucometer could be faulty. Having sugar in your urine is a sign of high blood sugar, so if your blood sugar reading is normal, it suggests that your test or meter is inaccurate in some way. If you continue to see seemingly implausible blood sugar readings, you may need to replace your glucometer.

Read more: 7 Foods That Won't Cause Blood Sugar Spikes

Other Causes of Sugar in Urine

A rare condition called renal glycosuria can cause a person to have sugar in their urine even if their blood sugar levels are normal. This condition occurs when the kidneys' filtration system doesn't work correctly and the kidneys eliminate glucose through urine, according to the National Organization for Rare Disorders 17.

Renal glycosuria rarely causes side effects. In fact, it is typically asymptomatic, meaning that it has no symptoms at all, and is therefore usually not a cause for concern.