Glucose in urine (glycosuria): causes, testing and follow-up

The kidneys filter glucose from blood and normally reabsorb all of it, so urine is glucose-free. Lab Tests Online UK explains that glucose will only show up in urine if it is at sufficiently high levels in the blood, above 10 mmol/L, for the body to excrete the excess, or if the kidneys are damaged and the glucose is leaking out. This is why urine glucose is described as a rough indicator of high blood glucose.

A reagent strip contains an enzyme reaction that changes colour in proportion to glucose, graded from negative upward, and results appear in about a minute, which is why dipsticks are used in clinics for quick screening.

Lab Tests Online UK says urine glucose testing is seldom requested; at one time it was used to monitor diabetes but has been largely replaced by the more sensitive real-time blood glucose. It is however one of the substances measured in a urinalysis, which may be done at a medical examination, a new pregnancy evaluation, when a urinary tract infection is suspected, or for other reasons.

GPnotebook states that a positive test for urinary glucose should always be followed by a random or fasting blood glucose to exclude diabetes mellitus. In antenatal care, glucose detected in urine may indicate undiagnosed gestational diabetes and prompt further testing, such as a 75 g oral glucose tolerance test.

A fresh urine sample is tested by dipstick. Lab Tests Online UK notes that glucose is better looked for in a specimen collected after a meal, whereas low levels of protein are better detected in a concentrated early morning sample. Pharmacy strips exist for urine glucose, ketones and pH, but the source says they are best performed by someone used to reading and interpreting them.

The commonest cause is diabetes, with blood glucose above the kidney threshold. The NHS lists thirst, passing urine more often, tiredness and weight loss as common symptoms and advises an urgent GP appointment or NHS 111 if you or your child has any of them. Other causes of raised blood glucose include acute stress, long-term kidney disease, Cushing syndrome, hyperthyroidism, pancreatitis and drugs such as corticosteroids, diuretics and oestrogens, as Lab Tests Online UK lists.

Sometimes glucose is in the urine with a normal blood level. This can happen in pregnancy, when kidney handling of glucose changes, and in a benign inherited condition called renal glycosuria. Lab Tests Online UK also says urine glucose and other reducing sugars may be tested when inherited metabolic diseases such as Fanconi syndrome or galactosaemia are suspected.

High glucose with symptoms such as being very thirsty, passing a lot of urine, stomach pain, deep breathing, fruity-smelling breath, drowsiness or confusion can mean diabetic ketoacidosis. The NHS says to check blood glucose and ketones if you have diabetes, and to call 999 or go to A&E for a high ketone level, or if you have these symptoms and have not been diagnosed with diabetes.

The follow-up blood test settles the question in most people. If diabetes is confirmed, the NHS says treatment depends on the type and is delivered by a diabetes team; type 1 needs insulin and type 2 may be helped by lifestyle changes and medicines. If glucose is normal in blood but present in urine repeatedly, your doctor may check kidney function and consider inherited causes.