SN4 SHORT NOTE 5M ★ High-yield

Define glycosuria. Classify its causes and describe the methods of detection.

Biochemistry · 1st Professional — Pre-Clinical

Definition

Glycosuria = the presence of detectable glucose in the urine.

Normally the urine contains less than 25 mg/dL, which routine tests do not detect

Renal Handling of Glucose

Glucose freely filtered at the glomerulus → Reabsorbed in the proximal convoluted tubule by SGLT-2 (90%) and SGLT-1 (10%) → Transport is saturable → Above the transport maximum (Tm) of about 350 mg/min → Glucose appears in urine

Renal threshold = 180 mg/dL — the blood level at which glucose first appears in urine
Splay — glycosuria begins a little below the calculated threshold because nephrons differ in their individual capacity

Classification of Causes

Type	Blood glucose	Causes
Hyperglycaemic glycosuria	Raised	Diabetes mellitus; also Cushing, acromegaly, phaeochromocytoma, thyrotoxicosis, severe stress, steroids
Renal glycosuria	Normal	Lowered renal threshold — familial renal glycosuria (SGLT-2 mutation), pregnancy, Fanconi syndrome, chronic kidney disease
Alimentary (lag storage)	Transient peak	After gastrectomy, in hyperthyroidism — rapid absorption overwhelms the threshold briefly
Drug-induced	Normal or low	SGLT-2 inhibitors — deliberate therapeutic glycosuria

CLINICAL PEARL

Glycosuria is not diabetes. A normal blood glucose with glycosuria means a low renal threshold, most often pregnancy. Conversely, the elderly and those with renal disease may have a raised threshold and show no glycosuria despite marked hyperglycaemia — which is why urine testing is unreliable for diagnosis.

Detection Methods

Method	Principle	Specificity
Glucose oxidase strip	Glucose oxidase → H2O2 → chromogen with peroxidase	Specific for glucose
Benedict test	Reduction of cupric to cuprous oxide in alkali	Detects any reducing sugar — glucose, galactose, fructose, lactose, pentose
Rothera test	Nitroprusside	For ketones, not sugar

A positive Benedict with a negative glucose oxidase strip means a non-glucose reducing sugar — suspect galactosaemia in an infant, or essential fructosuria, or lactosuria in late pregnancy. A classical examination question
False negatives on strips — large doses of vitamin C, or old strips
False positives on Benedict — ascorbic acid, salicylates, some antibiotics, homogentisic acid

Applied Aspects

Urine glucose is no longer used to diagnose or monitor diabetes — blood testing has replaced it entirely, because the renal threshold varies so widely between individuals
Glycosuria in pregnancy is common and usually physiological, from increased GFR and reduced tubular reabsorption; it still warrants a glucose tolerance test to exclude gestational diabetes
SGLT-2 inhibitors deliberately induce glycosuria of 50–80 g/day, lowering glucose independently of insulin; the cost is an increased risk of genital fungal infection and, rarely, euglycaemic ketoacidosis

All questions in Carbohydrate Metabolism II: Glycogen, HMP & Diabetes

Every Biochemistry chapter →