Kidney Function Tests Explained: eGFR, Creatinine & ACR

Your kidney function tests are usually three or four numbers at the bottom of a routine blood panel, and most men never look at them twice. That is a problem, because kidney damage stays close to silent until it is advanced. In clinic I regularly meet men in their fifties whose creatinine has drifted upward across four separate blood tests over six years — every one of them reported as normal — because nobody put the trend together. The panel itself is not hard to read once you know what each number measures and, more usefully, what each number cannot see. This article walks through eGFR, serum creatinine, blood urea nitrogen, cystatin C, and the urine albumin-to-creatinine ratio in the order I read them: what a genuinely reassuring set of results looks like, and which combination means you need a specialist opinion rather than a repeat test in twelve months. For the wider picture of how blood pressure damages kidneys over time, start with our blood pressure and kidney health hub.

What Kidney Function Tests Actually Measure

Each kidney holds roughly a million nephrons, and each nephron is a filter with a pressurized inlet. Blood arrives at a tight ball of capillaries called the glomerulus, water and small waste molecules are pushed across a three-layer barrier into a collecting cup, and the tubule downstream reclaims what the body still needs. Together your kidneys push around 180 liters (about 48 gallons) of fluid across that barrier every day and hand back all but a litre or two of it.

That design means there are only two questions worth asking of any lab panel. How fast is the filter working? That is glomerular filtration rate. Is the filter leaking? That is albuminuria. Every test on the panel is a proxy for one of those two things, and the reason so many men are falsely reassured is that most panels only answer the first question.

The barrier itself is what makes the leak question matter. A healthy glomerulus keeps albumin — a protein molecule far too large to pass — entirely inside the bloodstream. When high pressure or high glucose damages the podocytes lining that barrier, albumin starts crossing into the urine long before the filtration rate drops. This is exactly why uncontrolled blood pressure damages kidneys silently for years while the blood results stay unremarkable.

eGFR: The Number That Defines Your Kidney Stage

Estimated glomerular filtration rate is not measured directly. The lab takes your serum creatinine and runs it through an equation alongside your age and sex. Since 2021 most US and European labs use the race-free CKD-EPI equation published by Inker and colleagues in the New England Journal of Medicine, which removed the race coefficient that had been systematically overestimating filtration in Black patients [3].

KDIGO 2024 sorts the result into six bands [1]:

• G1 — 90 or above: normal filtration
• G2 — 60 to 89: mildly reduced
• G3a — 45 to 59: mild to moderate reduction
• G3b — 30 to 44: moderate to severe reduction
• G4 — 15 to 29: severely reduced
• G5 — below 15: kidney failure

Here is the misreading I correct most often. An eGFR between 60 and 89 with no other abnormality is not chronic kidney disease. KDIGO defines CKD as either an eGFR below 60, or a marker of kidney damage such as albuminuria, present for at least three months [1]. A man of 68 with an eGFR of 74, a clean urine ACR and no imaging abnormality has kidneys that are behaving their age. He does not have a disease, and he does not need a nephrology referral — he needs the same test repeated annually so the trajectory is visible.

The trajectory is the part that gets missed. A single eGFR is a snapshot; the slope across five years is the clinical information. A drop from 88 to 62 over four years matters far more than a steady 62.

Creatinine: Why a Normal Level Can Still Hide Kidney Damage

Creatinine is a waste product of muscle metabolism. Your muscles produce it at a fairly constant rate, your kidneys clear it, and the level in your blood settles at whatever balance those two processes reach. Typical adult male reference ranges run around 0.74 to 1.35 mg/dL (65 to 120 umol/L), though every lab publishes its own.

The trouble is that the relationship between creatinine and filtration is a curve, not a straight line. Filtration can fall substantially before creatinine climbs past the top of the reference range, which is why a man can lose a meaningful share of his filtering capacity while every blood test he has ever had comes back flagged as normal. That flat portion of the curve is the reason KDIGO 2024 pairs eGFR with a urine albumin test rather than relying on blood chemistry alone [2].

Muscle mass is the other distortion. A 110 kg (242 lb) man who lifts weights four times a week runs a genuinely higher baseline creatinine than a sedentary man of the same age at 70 kg (154 lb), and his eGFR will be underestimated as a result. The reverse is worse: an elderly, frail or amputee patient can carry a creatinine of 1.0 mg/dL with substantially reduced filtration, because there is not enough muscle to generate the creatinine that would reveal it.

Several things raise creatinine without touching kidney function at all — creatine supplements, a very high-protein diet in the days before the draw, trimethoprim, and cimetidine among them. If your number jumped and you started a supplement, retest after two weeks off it before anyone escalates. Our detailed breakdown of what a raised creatinine does and does not mean covers the full differential.

Urine Albumin-to-Creatinine Ratio: The Kidney Function Test Most Men Never Get

If you take one thing from this article, take this: the urine albumin-to-creatinine ratio is the single most underused test in men’s kidney health. It needs a spot urine sample, ideally first thing in the morning, and it costs a fraction of an imaging study. It detects filter damage while filtration itself is still normal.

KDIGO 2024 grades the result into three categories [1]:

• A1 — under 30 mg/g: normal to mildly increased
• A2 — 30 to 300 mg/g: moderately increased
• A3 — over 300 mg/g: severely increased

An A2 result with a completely normal eGFR still meets the definition of chronic kidney disease if it persists past three months, and it independently predicts cardiovascular events. That is the finding that changed how I counsel hypertensive and diabetic men: the ACR is not a kidney test in isolation, it is a vascular test that happens to be read in urine.

A standard urine dipstick will not do this job. Dipstick protein typically only turns positive once albumin excretion is already well into the A2 range or beyond, so a negative dipstick tells you almost nothing about early damage. If your doctor says your urine was clear, ask specifically whether an albumin-to-creatinine ratio was sent to the lab, not whether the dipstick was negative. We cover the interpretation in depth in our guide to protein in urine and what causes it.

One caveat before you panic at a single elevated ACR: heavy exercise in the previous 24 hours, fever, a urinary infection, and marked dehydration all push it up transiently. KDIGO advises confirming an abnormal result on a repeat sample rather than acting on one reading [2].

BUN, Cystatin C, and Urinalysis: The Supporting Tests

Blood urea nitrogen (BUN)

Urea is the nitrogen waste left over from protein breakdown. It is a poorer marker of filtration than creatinine because so many non-kidney things move it: dehydration raises it, a high-protein diet raises it, gastrointestinal bleeding raises it sharply, corticosteroids raise it, and liver disease lowers it. Its practical value is comparative. A BUN that has risen out of proportion to creatinine points toward volume depletion or a bleed rather than intrinsic kidney disease, and that distinction changes management on the same day.

Cystatin C

Cystatin C is a small protein produced by essentially every nucleated cell at a steady rate, and crucially it is not tied to muscle bulk. That makes it the confirmatory test when a creatinine-based eGFR does not fit the patient — the bodybuilder, the frail 82-year-old, the amputee, the man on a carnivore diet. Inker’s 2021 work showed that an equation combining creatinine and cystatin C estimates measured GFR more accurately than either marker alone [3], and KDIGO 2024 supports using it to confirm a borderline eGFR before labelling someone with CKD [1]. Ask for it by name — it is rarely on a default panel.

Urinalysis and microscopy

The dipstick screens for protein, blood, glucose, leukocyte esterase and nitrite. It is a triage instrument, not a diagnosis. The blood pad in particular is where I see the most confusion, because it reacts to free hemoglobin and myoglobin as well as intact red cells — so it can turn positive after a hard workout or a muscle injury with no bleeding at all.

This is why the AUA and SUFU are explicit that microhematuria is defined as more than 3 red blood cells per high-power field on microscopy of a single properly collected specimen, and specifically not by a positive dipstick alone [4]. A positive dipstick needs microscopy to confirm it before anyone books a cystoscopy. Once it is confirmed, the evaluation follows a risk-stratified pathway rather than a one-size-fits-all workup — our article on blood in urine and its causes in men walks through that assessment.

How to Read Your Kidney Function Tests Together

Neither number means much alone. KDIGO stages risk on a grid: eGFR band down one axis, albuminuria category across the other. A man at G2 with A1 is at low risk. A man at G2 with A3 is at high risk despite a filtration rate that looks nearly normal on paper. A man at G3a with A1 sits somewhere between. The combination is the answer, which is why ordering only half of it produces so much false reassurance [1].

Three rules I apply to every panel that crosses my desk:

• Three months, not three minutes. One abnormal eGFR or ACR is an observation. Chronic kidney disease requires persistence for at least 90 days, so the correct response to a first abnormal result is usually a repeat, not a referral.
• Read the slope, not the point. Ask your doctor to print every creatinine and eGFR on file going back five years. A steady 63 is a different clinical situation from a 63 that was 89 four years ago.
• Match the test to the body. If your muscle mass is unusual in either direction, a creatinine-based eGFR alone is the wrong instrument. Request cystatin C.

What to actually ask for at your next appointment: an eGFR and a urine albumin-to-creatinine ratio drawn on the same day, plus a printout of your previous creatinine values. If either result is abnormal, ask for both to be repeated in 8 to 12 weeks before any label is applied. If you already know your stage and want to understand what follows, our guide to the stages of chronic kidney disease sets out what changes at each band.

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Dr. Muhammad Khalid

MBBS · FCPS (Urology) · MCPS (Gen. Surgery) · CHPE · CRSM · IMC #539472

Specialist urologist with 11+ years of clinical experience across tertiary teaching hospitals. Trained at Lady Reading Hospital and Khyber Teaching Hospital, Peshawar. Author of 5 peer-reviewed international publications in Cureus, WJSA, and AJBS. Procedural expertise: URS, PCNL, RIRS, TURP, TURBT, and major open urological surgery. Full profile →

This article is for educational purposes only and does not constitute medical advice. Always consult your physician or urologist for diagnosis and treatment decisions specific to your condition.