Age-related fall in glomerular filtration rate
Normal ageing steadily removes glomeruli (the kidney's tiny filters), so the glomerular filtration rate (GFR), the volume of fluid the kidneys filter each minute, falls gradually even without kidney disease.
Remember
The GFR falls by roughly 1% a year after the age of 30 because glomeruli are lost, a gradual change of normal ageing.
Memory aid
Think of a sieve losing a few holes every year from the age of 30: each hole lost is a glomerulus gone, so the total amount filtered slowly falls.
Each hole lost from the sieve stands for one glomerulus gone, so less is filtered.
The kidneys filter blood in glomeruli, tiny clusters of capillaries (the smallest blood vessels) where blood is first filtered, at the start of each nephron, the kidney's working unit. The renal cortex is the outer layer of the kidney. Kidney function is measured by the glomerular filtration rate (GFR), the volume of fluid the kidneys filter each minute.
With normal ageing, glomeruli are lost throughout the renal cortex. Fewer working filters means a smaller total filtration surface, so the GFR falls. A useful rule of thumb is a fall of about 1% a year after the age of 30. The decline is gradual and happens even when there is no kidney disease, although it is faster in diabetes, hypertension and with some drugs.
Reduced blood supply adds to the loss. Vascular insufficiency, meaning poorer blood supply with reduced renal perfusion (blood flow through the kidneys), promotes glomerulosclerosis, which means scarring of the glomeruli. Both cut filtration further.
A modestly reduced GFR in an older person does not always mean kidney disease, but it matters for drug doses. Many drugs are cleared by the kidneys, so doses of such drugs may need to be lower in older adults. Non-steroidal anti-inflammatory drugs can harm the kidneys, so older patients taking them need monitoring.
Key facts
- Rate
- the GFR falls by approximately 1% each year after the age of 30.
- Cause
- glomeruli are lost throughout the renal cortex.
- Pattern
- the decline is gradual and very variable between people, and it does not suddenly start in old age.
- Blood flow
- renal perfusion falls with age, which further reduces filtration.
- Scarring
- glomerulosclerosis, the scarring of glomeruli, becomes more common with age.
- Prescribing
- doses of kidney-cleared drugs may need to be lower in older adults.
Exam traps
- Kidney function does not stay stable until late old age and then decline rapidly, because the decline is gradual from early adulthood, and a sudden fall in old age points to an acute problem rather than normal ageing.
- The fall in GFR of about 1% a year starts after the age of 30, not after 50, because glomeruli are lost throughout adult life.
- Nephron hypertrophy (enlargement of the remaining nephrons) can occur when nephrons are lost, but it cannot make the overall GFR rise, so the GFR still falls with age.
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