Kidney Function Tests: Creatinine, Urea, Uric Acid, and eGFR

Kidneys filter roughly 180 litres of fluid a day and return almost all of it, adjusting what stays and what leaves. The renal panel does not measure that filtration directly. It measures waste products the kidneys are responsible for clearing, and infers filtration from how much has built up.

Creatinine

Creatinine is a waste product of normal muscle metabolism, produced at a fairly steady rate and cleared almost entirely by the kidneys. That steadiness is what makes it useful: if production is constant, the blood level reflects clearance.

The steadiness is also its limitation. Creatinine production depends on muscle mass, so a heavily muscled adult and a frail elderly person with identical kidney function will not produce identical creatinine values. This is why creatinine is interpreted with body context rather than against a single universal number.

Creatinine is also relatively insensitive early on. The kidneys have substantial functional reserve, and a meaningful proportion can be lost before the blood level moves outside its reference range.

eGFR — a calculation, not a measurement

Estimated glomerular filtration rate is the number most reports lead with, and it is worth knowing that no instrument measures it. eGFR is calculated from serum creatinine together with age and sex, using a published equation such as CKD-EPI.

Two consequences follow. First, anything that distorts creatinine distorts eGFR identically. Second, because equations have been revised over the years — including the 2021 CKD-EPI revision that removed a race coefficient — an older eGFR and a newer one for the same creatinine may differ purely because the laboratory changed its formula. When comparing across years, it is worth checking which equation each report used.

Urea and blood urea nitrogen

Urea is the nitrogen waste produced when the liver breaks down protein. Indian reports usually give blood urea; many international ones give blood urea nitrogen (BUN), which measures only the nitrogen portion. The two are not interchangeable — BUN is roughly urea divided by 2.14 — and this is a frequent source of confusion when comparing reports from different sources.

Urea is a less specific marker of kidney function than creatinine because it responds to a wider set of influences:

• Dietary protein intake
• Hydration state — urea is reabsorbed more when the body is conserving water
• Gastrointestinal bleeding, where digested blood acts as a protein load
• Liver function, since the liver produces urea in the first place

That breadth is exactly why urea and creatinine are read together: their ratio can distinguish reduced kidney filtration from causes upstream of the kidney, such as dehydration.

Uric acid

Uric acid is the end product of purine breakdown, also cleared largely by the kidneys. It appears on renal panels for two separate reasons: it rises when clearance falls, and independently it can crystallise in joints and in the urinary tract. Reference ranges differ between men and women.

Why urine tests are ordered alongside

Blood markers describe what has accumulated. Urine tests describe what is leaking. Protein or albumin in urine — often reported as the urine albumin-to-creatinine ratio — can signal kidney damage while blood creatinine and eGFR still look unremarkable, which is why the two sets of tests complement rather than duplicate each other.