Kidneys work quietly. They filter the blood, balance fluids and salts and remove waste, and the NHS notes that there are usually no symptoms of kidney disease in the early stages. Most changes in kidney function are found when a blood or urine test is done for another reason. This guide explains the main kidney function tests, what eGFR and urine ACR mean, when cystatin C helps and who tends to benefit from checking.
Two questions, two kinds of test
UK guidance from NICE assesses kidney health with two measures used together:
- How well the kidneys filter, estimated from a blood test (eGFR).
- Whether the filter is leaking protein, measured in urine (the albumin:creatinine ratio, or ACR).
NICE classifies chronic kidney disease (CKD) using both, because each adds information the other does not. The KDIGO 2024 international guideline uses the same two-axis approach (KDIGO CKD Work Group, 2024).
Creatinine and eGFR
Creatinine is a waste product from normal muscle activity. Healthy kidneys clear it steadily, so when filtering slows, creatinine in the blood tends to rise. On its own, creatinine is hard to interpret, which is why laboratories convert it into an estimated glomerular filtration rate (eGFR) using age and sex.
LML's description of the eGFR test gives the usual bands, in mL/min/1.73 m²:
- Above 90: normal kidney function.
- 60 to 89: mildly decreased.
- 30 to 59: moderately decreased.
- 15 to 29: severely decreased.
- Below 15: kidney failure.
LML adds that eGFR can be affected by age, sex, muscle mass and some medicines, and should always be read with your history.
Things that can shift the number
NICE highlights a few practical points:
- Muscle mass: eGFR from creatinine should be read with caution in people with very high or very low muscle mass, such as bodybuilders, people who have had an amputation or people with muscle-wasting conditions. More muscle tends to underestimate filtering; less muscle tends to overestimate it.
- Meat: NICE advises adults not to eat any meat in the 12 hours before a creatinine-based eGFR test.
- Protein supplements and pregnancy can also make creatinine-based eGFR less reliable.
- A single low result: NICE recommends repeating an eGFR below 60 within 2 weeks in someone not tested before, and allowing for natural variation of about 5% in creatinine. CKD is only considered when eGFR stays below 60 on at least two occasions at least 90 days apart.
Cystatin C: a second marker for filtering
Cystatin C is a protein made by cells throughout the body and filtered by the kidneys. LML describes the test as a way to evaluate kidney function, with raised levels suggesting reduced filtering. Because it depends much less on muscle, it is useful when creatinine may mislead, for example in athletes, people who train hard or use protein supplements, and older adults with less muscle.
A study in the New England Journal of Medicine developed new equations for eGFR and found that equations combining creatinine and cystatin C were more accurate than those using creatinine alone (Inker et al., 2021). The KDIGO 2024 guideline also supports cystatin C when a more accurate estimate is needed.
Urine albumin:creatinine ratio (ACR)
Albumin is a protein that healthy kidneys keep in the blood. When the filter is under strain, small amounts leak into the urine. The ACR compares albumin with creatinine in a single urine sample, which corrects for how concentrated the urine is.
NICE regards a confirmed ACR of 3 mg/mmol or more as clinically important, and recommends confirming results between 3 and 70 mg/mmol with an early morning urine sample. LML describes urine microalbumin as particularly important for people with diabetes, where it is used to look for kidney involvement.
Large pooled analyses by the CKD Prognosis Consortium show that both lower eGFR and higher albuminuria are linked to a range of adverse outcomes, which is why both are measured (Writing Group for the CKD Prognosis Consortium, 2023).
