Kidney Function Test — Test Guide & Results Explained — Purpose, Procedure & Normal Values | MyMedicPlus

Kidney function tests (KFTs) — also called renal function tests (RFTs), a renal panel, or a urea and electrolytes (U&E) panel — are a group of coordinated blood and urine tests that comprehensively assess how efficiently the kidneys perform their primary roles: filtering nitrogenous metabolic waste products from the blood, regulating fluid and electrolyte balance (sodium, potassium, chloride, bicarbonate), maintaining acid-base homeostasis, activating vitamin D to its active form (calcitriol), and producing erythropoietin to stimulate red blood cell production. The standard blood panel includes serum creatinine — a breakdown product of muscle creatine phosphate produced at a relatively constant rate, making it the most specific marker of glomerular filtration — and blood urea nitrogen (BUN), derived from dietary protein catabolism. From the serum creatinine, sex, and age (and previously race in older CKD-EPI equations), laboratories automatically calculate the estimated glomerular filtration rate (eGFR) using the CKD-EPI 2021 equation, expressed in mL/min/1.73m². Electrolytes (sodium, potassium, chloride, bicarbonate) assess tubular function and acid-base status. Urine tests extend the assessment: a urine albumin-to-creatinine ratio (ACR) detects early kidney damage in the form of microalbuminuria before creatinine rises; urinalysis identifies haematuria, pyuria, and casts; and a 24-hour urine collection provides creatinine clearance and total protein excretion. Together, these tests diagnose, stage, and monitor kidney disease.

KFTs are ordered to diagnose acute kidney injury (AKI) — a rapid decline in kidney function over hours to days — and chronic kidney disease (CKD), a sustained reduction in eGFR below 60 mL/min/1.73m² or the presence of kidney damage markers (proteinuria, haematuria, structural abnormalities) for more than 3 months. They monitor CKD progression and guide referral to nephrology (typically when eGFR falls below 30). A fundamental safety indication is checking kidney function before initiating or continuing potentially nephrotoxic medications including NSAIDs (ibuprofen, naproxen), aminoglycoside antibiotics, calcineurin inhibitors (tacrolimus, cyclosporine), lithium, methotrexate, and iodinated radiological contrast medium. In diabetes and hypertension — the two leading causes of CKD worldwide — annual KFTs identify early kidney involvement that can be slowed with renin-angiotensin-aldosterone system (RAAS) blockade and tight blood pressure and glycaemic control. Symptoms prompting KFTs include unexplained oedema (ankle swelling), oliguria (reduced urine output), haematuria (blood in urine), loin or flank pain, hypertension, generalised fatigue, and uraemic symptoms (nausea, anorexia, pruritus, cognitive impairment, pericarditis) in advanced renal failure.

A standard KFT blood panel does not require fasting, but some laboratories request 4–8 hours of fasting to avoid the transient rise in BUN and creatinine that follows a high-protein meal or vigorous exercise. Maintain your normal fluid intake — adequate hydration is important because dehydration raises creatinine and BUN, potentially overstaging kidney disease. Do not perform heavy resistance exercise or intense cardiovascular training in the 24 hours before the blood test, as muscle breakdown releases large amounts of creatinine that falsely elevate the serum level and reduce the calculated eGFR. Inform your doctor of all medications, particularly those that raise creatinine without impairing GFR (trimethoprim and cimetidine block tubular creatinine secretion, raising serum creatinine without affecting true GFR). For a first-morning urine albumin-to-creatinine ratio (ACR) test, collect the first urine specimen of the day in a clean, sterile container provided by the laboratory before any other fluid intake, as concentrated morning urine gives the most reliable albumin measurement.

A nurse or phlebotomist applies a tourniquet, cleans the antecubital fossa with antiseptic, and draws 5–10 mL of blood from a forearm vein into one or two collection tubes — typically a serum separator (gold or red-top) tube for the biochemistry panel and a lithium heparin (green-top) tube for electrolytes. The sample is transported to the biochemistry laboratory where automated analysers measure serum creatinine enzymatically (Jaffe or enzymatic method), BUN by urease-glutamate dehydrogenase reaction, and electrolytes by ion-selective electrodes. The laboratory computer automatically applies the 2021 CKD-EPI creatinine equation to calculate eGFR. For a 24-hour urine creatinine clearance test, you collect every urine void over exactly 24 hours in a large labelled container, noting the total volume before delivering it to the laboratory. Results from a blood panel are available within 2–8 hours; urine ACR from a spot sample is available within 4–24 hours.

Blood test normal ranges: Serum creatinine — Men 0.7–1.3 mg/dL (62–115 μmol/L); Women 0.5–1.1 mg/dL (44–97 μmol/L). Blood urea nitrogen (BUN) — 7–20 mg/dL (2.5–7.1 mmol/L). eGFR — ≥90 mL/min/1.73m² is normal; CKD Stage 1 (G1): eGFR ≥90 with kidney damage markers (proteinuria, haematuria); G2: 60–89; G3a: 45–59; G3b: 30–44; G4: 15–29 (prepare for renal replacement therapy); G5 (kidney failure): eGFR <15 requiring dialysis or transplantation. Sodium — 136–145 mmol/L; Potassium — 3.5–5.0 mmol/L; Bicarbonate — 22–29 mmol/L; Chloride — 98–107 mmol/L. BUN/creatinine ratio above 20:1 suggests pre-renal causes (dehydration, heart failure, GI haemorrhage); normal ratio with elevated creatinine suggests intrinsic renal disease. Urine ACR — Normal <30 mg/g (A1 category); Moderately increased 30–300 mg/g (A2, microalbuminuria, indicates early diabetic nephropathy or early CKD); Severely increased ≥300 mg/g (A3, macroproteinuria — significant kidney damage requiring investigation and RAAS blockade). Acidosis (bicarbonate <22 mmol/L) with elevated creatinine indicates significant CKD-related metabolic acidosis requiring bicarbonate supplementation.

The blood draw is very low risk — minor bruising, soreness, or a small haematoma at the venepuncture site are the only common adverse effects. Creatinine-based eGFR calculated by CKD-EPI has important limitations in specific patient populations. It overestimates GFR (underestimates disease severity) in patients with very low muscle mass — malnutrition, cachexia, amputees, sarcopenic elderly, or vegetarians — because creatinine production is reduced by reduced muscle bulk. Conversely, it underestimates GFR (falsely suggests worse renal function) in highly muscular athletes and bodybuilders. In these populations, cystatin C — an endogenous protease inhibitor produced at a constant rate by all nucleated cells and independent of muscle mass — provides a more accurate filtration marker; the CKD-EPI cystatin C or combined creatinine-cystatin C equation is preferred. A single creatinine result cannot distinguish between acute kidney injury (potentially reversible) and chronic kidney disease (irreversible fibrosis) — only comparison with baseline measurements over months reveals the timeline. Very mild creatinine elevation (1.4–1.8 mg/dL) in an elderly patient may represent substantial renal reserve loss without reaching diagnostic CKD thresholds, because elderly patients have low muscle mass and generate less creatinine than younger adults. AKI from dehydration, contrast, or medication is often reversible — do not diagnose CKD without confirmed persistence of abnormalities for at least 3 months.