Overview.
The kidney function panel evaluates glomerular filtration (eGFR, creatinine, cystatin C), tubular function (BUN:creatinine ratio), and early kidney damage (urine albumin:creatinine ratio). Chronic kidney disease affects approximately 15% of US adults but most are undiagnosed because early stages are completely asymptomatic. eGFR — estimated from creatinine (and optionally cystatin C) — is the cornerstone metric, staging CKD from mild impairment to kidney failure using the KDIGO classification.
Glomerular filtration rate (estimated from serum creatinine ± cystatin C), nitrogen waste clearance (BUN), and urine albumin excretion (UACR). The combination of eGFR and albuminuria together determines CKD stage and prognosis using the KDIGO heat map — eGFR alone is insufficient for complete staging.
What's included.
- Serum Creatinine — normal: M: 0.7-1.3 / F: 0.6-1.1 mg/dL · Muscle breakdown product filtered by kidneys. Rising creatinine = falling kidney function
- eGFR (Estimated Glomerular Filtration Rate) — normal: >90 mL/min/1.73m² · The headline kidney number. <60 sustained for 3+ months = CKD. Uses CKD-EPI 2021 equation
- BUN (Blood Urea Nitrogen) — normal: 7-20 mg/dL · Protein breakdown waste. Elevated in dehydration, high-protein diet, GI bleeding, kidney disease
- BUN:Creatinine Ratio — normal: 10:1 to 20:1 ratio · >20:1 = pre-renal cause (dehydration, bleeding). <10:1 = liver disease or low protein
- Cystatin C — normal: 0.6-1.0 mg/L · Not affected by muscle mass — more accurate eGFR in bodybuilders, amputees, elderly, vegetarians
- Urine Albumin:Creatinine Ratio (UACR) — normal: <30 mg/g · Early kidney damage marker. 30-300 = moderately elevated. >300 = severely elevated (nephrotic range)
Preparation.
No fasting required.
When: Can be drawn at any time. For UACR, first morning void is most accurate but random spot urine is acceptable. Results typically available within 24 hours.
- No fasting required for creatinine/eGFR
- Stay normally hydrated — dehydration falsely elevates creatinine and BUN
- Avoid creatine supplements for 48 hours before testing (falsely elevates creatinine)
- Note that intense exercise within 24 hours can elevate creatinine
- UACR requires a spot urine sample (first morning void preferred)
- Certain medications affect creatinine without affecting kidney function (trimethoprim, cimetidine)
When it's ordered.
- Diabetes mellitus (annual screening recommended)
- Hypertension (annual screening recommended)
- Elevated creatinine found on routine metabolic panel
- Family history of kidney disease or kidney failure
- Long-term NSAID or nephrotoxic medication use
- Systemic conditions affecting the kidneys (lupus, vasculitis)
- Recurrent urinary tract infections or haematuria
- Heart failure (cardiorenal monitoring)
- Age >60 with any cardiovascular risk factor
Interpretation.
What normal means
Kidneys are filtering blood at a normal rate and not leaking significant protein. eGFR >90 with UACR <30 = no evidence of kidney disease. Note: eGFR naturally declines with age — an eGFR of 70 in an 80-year-old may be age-appropriate.
Abnormal patterns
eGFR 60-89 with normal UACR (CKD Stage G2 — mild decrease)
Possible causes
- Age-related decline (may be normal in elderly)
- Early hypertensive nephropathy
- Resolved acute kidney injury
Next steps
- Repeat in 3 months to confirm (must be sustained ≥3 months for CKD diagnosis)
- If stable and UACR normal: low risk — annual monitoring
- Optimise blood pressure (<130/80 mmHg per KDIGO)
- Avoid nephrotoxic medications where possible
eGFR 30-59 (CKD Stage G3a/G3b — moderate decrease)
Possible causes
- Diabetic kidney disease
- Hypertensive nephrosclerosis
- Glomerulonephritis
- Polycystic kidney disease
- Chronic NSAID use
Next steps
- Nephrology referral if eGFR <45 or declining rapidly (>5 mL/min/year)
- Check UACR if not already done
- Medication dose adjustments (metformin, DOACs, antibiotics)
- Avoid contrast dye without pre-hydration
- Consider cystatin C-based eGFR if muscle mass confounders exist
eGFR <30 (CKD Stage G4-G5 — severe decrease to kidney failure)
Possible causes
- Advanced CKD from any cause
- Approaching need for renal replacement therapy
Next steps
- Urgent nephrology referral
- Renal replacement planning (dialysis, transplant evaluation)
- Monitor potassium, phosphate, PTH, haemoglobin
- Erythropoietin for renal anaemia if Hb <10 g/dL
Normal eGFR but elevated UACR (>30 mg/g)
Possible causes
- Early diabetic kidney disease (before eGFR drops)
- Hypertensive nephropathy
- Glomerulonephritis
- Fever or vigorous exercise (transient elevation)
Next steps
- Repeat UACR to confirm (rule out transient causes)
- If persistently elevated: ACE inhibitor or ARB initiation (renoprotective)
- Tight glucose control in diabetes
- SGLT2 inhibitor consideration (renoprotective regardless of diabetes)
BUN:Creatinine ratio >20:1 with normal eGFR (Pre-renal pattern)
Possible causes
- Dehydration (most common)
- Upper GI bleeding
- High-protein diet
- Heart failure (reduced renal perfusion)
Next steps
- Rehydrate and repeat in 48-72 hours
- If persistent: evaluate cardiac function and GI tract
Cost & access.
Basic renal panel (creatinine, BUN, eGFR, electrolytes) is included in CMP ($20-50). Adding cystatin C adds $30-80. Urine albumin:creatinine ratio (UACR) is a separate urine test ($20-50). Most insurance covers with clinical indication.
FAQs.
What does eGFR actually measure?
eGFR estimates how many millilitres of blood your kidneys filter per minute. It's calculated from serum creatinine (and optionally cystatin C) using the CKD-EPI 2021 equation, which accounts for age and sex. Normal is >90 mL/min/1.73m². Below 60 sustained for 3+ months = CKD.
I'm muscular / a bodybuilder — is my creatinine unreliable?
Yes. Creatinine is produced by muscle metabolism, so high muscle mass elevates creatinine without kidney impairment — making eGFR appear falsely low. Request cystatin C-based eGFR, which is not affected by muscle mass, for a more accurate assessment.
Why is UACR important if my eGFR is normal?
Albuminuria (protein leak) is often the earliest sign of kidney damage — appearing before eGFR drops. In diabetes and hypertension, UACR can be elevated for years while eGFR remains normal. Catching this early allows intervention (ACE inhibitors, SGLT2 inhibitors) that slows or prevents progression.
Can kidney function improve once it's declined?
Acute causes (dehydration, medication effects, obstruction) are often reversible. Chronic kidney disease from diabetes or hypertension is generally not reversible, but progression can be significantly slowed with blood pressure control, SGLT2 inhibitors, and avoiding nephrotoxins. Early detection makes the biggest difference.