Clinical focus: This single-page guide teaches students to interpret renal function tests in a structured way—from specimen quality and individual analytes to complete AKI/CKD patterns and emergency decisions. Reference intervals vary by laboratory, age, sex, pregnancy and clinical context; use the local laboratory range and the patient’s baseline whenever available.
Learning objectives
- Explain what the kidneys measure and why no single “kidney test” is sufficient.
- Interpret creatinine, urea/BUN, eGFR, creatinine clearance, cystatin C, electrolytes and acid–base results.
- Read urinalysis, albumin–creatinine ratio (ACR), protein–creatinine ratio (PCR) and urinary sediment.
- Recognise and stage acute kidney injury (AKI), distinguish pre-renal, intrinsic and post-renal patterns, and identify chronic kidney disease (CKD).
- Recognise nephritic, nephrotic, tubular and obstructive patterns, adjust medication risk, and escalate renal emergencies.
1. What renal function means
The kidneys maintain internal homeostasis by filtering plasma, selectively reabsorbing and secreting solutes, excreting metabolic waste and drugs, regulating water, sodium, potassium, calcium and phosphate, maintaining acid–base balance, and producing or activating hormones (erythropoietin, renin and calcitriol). Renal tests therefore assess several domains:
| Domain | What to assess | Typical tests |
|---|---|---|
| Filtration | How much plasma is filtered per minute | Serum creatinine, eGFR, measured creatinine clearance, cystatin C |
| Glomerular barrier | Leakage of albumin, other proteins or blood | Urine ACR/PCR, dipstick, microscopy |
| Tubular function | Concentration, dilution and selective handling of solutes | Urine osmolality, specific gravity, urinary sodium, casts, fractional excretions |
| Excretion and homeostasis | Waste, potassium, bicarbonate and fluid balance | Urea/BUN, K+, Na+, HCO3−, phosphate, calcium, fluid balance |
| Endocrine function | Erythropoiesis, blood pressure and bone/mineral regulation | Haemoglobin, renin/aldosterone and calcium–phosphate–PTH profile when indicated |
2. Before interpreting: verify the clinical and laboratory context
- Confirm patient identity, sample type, collection time and whether results are comparable with the same laboratory.
- Find the baseline creatinine and trend over hours to days, not just the latest value. A “normal” creatinine may be dangerous in a patient with very low muscle mass.
- Record weight, urine output (hourly in an unstable patient), blood pressure, volume status, fever, sepsis, bleeding, vomiting/diarrhoea, heart failure and urinary symptoms.
- Review drugs and exposures: NSAIDs, ACE inhibitors/ARBs, diuretics, aminoglycosides, vancomycin, amphotericin, tenofovir, chemotherapy, herbal medicines and recent iodinated contrast.
- Interpret results alongside glucose, liver tests, full blood count, CK, lactate, blood gas and imaging when clinically indicated.
3. Serum filtration markers
Serum creatinine
Creatinine is generated from muscle creatine and released relatively steadily into blood. It is filtered at the glomerulus and only minimally secreted, so a rising concentration usually indicates reduced filtration. It is a lagging marker: a major fall in GFR can occur before serum creatinine rises, especially during rapidly evolving AKI.
| Creatinine result | Possible explanation | Interpretive caution |
|---|---|---|
| High | Reduced GFR, dehydration/pre-renal state, obstruction, rhabdomyolysis, high muscle mass or creatine intake | Trimethoprim and cimetidine reduce tubular secretion; assay interference may occur with ketones or bilirubin. |
| Low or “normal” | Low muscle mass, malnutrition, amputation, pregnancy or severe liver disease | May conceal severe kidney dysfunction; assess eGFR trend, urine findings and clinical state. |
| Rapid rise | AKI until proven otherwise | Repeat, quantify urine output and search for reversible causes immediately. |
Estimated glomerular filtration rate (eGFR)
eGFR is calculated from creatinine (and sometimes cystatin C) with demographic variables. It is more useful than creatinine alone for stable outpatient CKD, but it is not a directly measured value and is unreliable when kidney function is changing quickly.
| G category | eGFR (mL/min/1.73 m2) | Meaning |
|---|---|---|
| G1 | ≥90 | Normal or high; CKD only if another marker of kidney damage persists. |
| G2 | 60–89 | Mildly decreased; not CKD without persistent albuminuria, structural disease or another marker. |
| G3a | 45–59 | Mild-to-moderate decrease. |
| G3b | 30–44 | Moderate-to-severe decrease; medication review is essential. |
| G4 | 15–29 | Severely decreased; prepare renal replacement planning and specialist care. |
| G5 | <15 | Kidney failure; assess symptoms and dialysis/transplant needs, not the number alone. |
When eGFR is less reliable: rapidly changing creatinine (AKI), extremes of muscle mass, amputation, severe malnutrition, oedema or pregnancy, unusual diets, and some medications. Consider a combined creatinine–cystatin C estimate, a measured GFR, or a carefully collected creatinine clearance when a precise value changes management. Do not use an indexed eGFR blindly for drug dosing in a very large or small body size; follow the medicine’s dosing guidance.
Creatinine clearance
With a timed urine collection, creatinine clearance can be estimated as:
CrCl (mL/min) = (urine creatinine × urine flow rate) ÷ plasma creatinine. The collection must be complete and timed accurately. Tubular secretion can make CrCl overestimate true GFR, and a missed urine collection makes the result meaningless. The Cockcroft–Gault equation is used by some drug labels but should not be substituted automatically for the laboratory eGFR.
Urea/BUN and the BUN:creatinine relationship
Urea is produced by hepatic protein metabolism and is filtered, then variably reabsorbed. It rises with reduced filtration but also with gastrointestinal bleeding, high protein intake, catabolism, fever, corticosteroids or dehydration. Low urea can occur with severe liver dysfunction, low protein intake or overhydration.
| Pattern | Common interpretation |
|---|---|
| Urea rises out of proportion to creatinine | Pre-renal hypoperfusion, upper GI bleeding, catabolism or steroids; interpret with volume status, not as proof of dehydration. |
| Creatinine rises with modest urea change | Intrinsic renal injury, low protein intake or reduced hepatic urea production may contribute. |
| Both high | Reduced filtration is likely; determine acute versus chronic and identify the cause. |
The traditional BUN:creatinine ratio is unit- and assay-dependent and is supportive only. It must never override urine findings, haemodynamics or the creatinine trend.
Cystatin C
Cystatin C is produced by nucleated cells, filtered freely and less dependent on muscle mass than creatinine. It may improve GFR estimation in frailty or unusual body composition. Inflammation, thyroid disease, corticosteroids and some other conditions can alter cystatin C; it is not a perfect AKI marker. A combined creatinine–cystatin C eGFR can improve confidence when values are discordant.
4. Urine tests: the kidney’s “microscope”
Dipstick and physical examination
| Finding | What it suggests | Important limitations |
|---|---|---|
| Specific gravity | Urine concentration; persistently fixed around 1.010 may indicate poor concentrating ability | Glucose, protein, contrast and radiographic substances alter readings. |
| Protein | Albumin or other protein leakage | Dipstick is most sensitive to albumin and may miss light chains; confirm with ACR/PCR. |
| Blood | Haematuria, haemoglobin or myoglobin | Positive blood with few/no RBCs suggests haemoglobinuria or myoglobinuria. |
| Leukocyte esterase/nitrite | Pyuria or nitrate-reducing bacteria | False negatives occur with dilute urine, non-nitrate-reducing organisms or early infection. |
| Glucose/ketones | Hyperglycaemia, tubular dysfunction or starvation/ketosis | Check blood glucose and beta-hydroxybutyrate if acidosis or illness is present. |
Albumin–creatinine ratio (ACR) and protein–creatinine ratio (PCR)
ACR on a spot urine sample corrects approximately for urine concentration and is preferred for detecting albuminuria. Confirm an unexpected elevation using a first-morning sample because exercise, fever, urinary infection, uncontrolled blood pressure, menstruation and heart failure can cause transient albuminuria.
| Albuminuria category | ACR | Risk meaning |
|---|---|---|
| A1 | <30 mg/g (<3 mg/mmol) | Normal to mildly increased. |
| A2 | 30–300 mg/g (3–30 mg/mmol) | Moderately increased; persistent findings indicate kidney damage. |
| A3 | >300 mg/g (>30 mg/mmol) | Severely increased; high progression and cardiovascular risk. |
PCR estimates total protein and is useful when non-albumin proteins or heavy proteinuria are suspected. Nephrotic-range protein loss is approximately >3–3.5 g/day (or equivalent PCR), but quantify locally and interpret with serum albumin, oedema and lipid profile.
Microscopy and casts
| Urine sediment | Likely implication |
|---|---|
| Dysmorphic RBCs or RBC casts | Glomerular bleeding (nephritic syndrome or glomerulonephritis). |
| WBCs or WBC casts | Pyelonephritis or tubulointerstitial nephritis; sterile pyuria needs broader differential. |
| Muddy brown granular casts | Acute tubular injury/necrosis. |
| Fatty casts or oval fat bodies | Heavy glomerular protein loss, often nephrotic syndrome. |
| Broad waxy casts | Advanced chronic kidney disease and urinary stasis. |
| Hyaline casts | May be benign in concentration, exercise or dehydration; correlate clinically. |
| Crystals | Stone disease, drug toxicity or metabolic disease depending on crystal type and pH. |
5. Electrolytes, bicarbonate and related results
- Potassium: reduced renal excretion causes hyperkalaemia; repeat a high value if haemolysis is suspected, but do not delay treatment when ECG changes or severe symptoms are present.
- Bicarbonate: low HCO3− may indicate metabolic acidosis from reduced acid excretion, bicarbonate loss, lactic acidosis or ketoacidosis. Use a blood gas and calculate the anion gap when clinically important.
- Sodium and water: interpret with fluid balance, urine sodium/osmolality, glucose and diuretic use. Hyponatraemia is not automatically a renal failure result.
- Phosphate, calcium, PTH and vitamin D: persistent CKD can cause phosphate retention, low calcitriol, secondary hyperparathyroidism and altered calcium balance.
- Magnesium: can accumulate in advanced CKD, particularly with magnesium-containing antacids or laxatives.
Anion gap: AG = Na+ − (Cl− + HCO3−). A raised gap indicates unmeasured anions (for example lactate, ketones or uraemic acids); correct interpretation requires albumin and clinical context.
6. Acute kidney injury (AKI)
Use the KDIGO framework: AKI is a rise in serum creatinine of ≥0.3 mg/dL (≥26.5 micromol/L) within 48 hours, or ≥1.5 times baseline within 7 days, or urine output <0.5 mL/kg/hour for 6 hours. Creatinine and urine output criteria may disagree; use the more severe stage and investigate both.
| Stage | Serum creatinine criterion | Urine-output criterion |
|---|---|---|
| 1 | 1.5–1.9× baseline or increase ≥0.3 mg/dL | <0.5 mL/kg/h for 6–12 h |
| 2 | 2.0–2.9× baseline | <0.5 mL/kg/h for ≥12 h |
| 3 | ≥3× baseline, or creatinine ≥4.0 mg/dL, or renal replacement therapy started | <0.3 mL/kg/h for ≥24 h or anuria ≥12 h |
Classify the mechanism: pre-renal, intrinsic or post-renal
| Category | Examples | Helpful clues | Immediate priorities |
|---|---|---|---|
| Pre-renal hypoperfusion | Haemorrhage, vomiting/diarrhoea, sepsis, burns, heart failure, cirrhosis | History of losses, hypotension, concentrated urine; bland sediment | Restore perfusion carefully, stop offending drugs, monitor response and avoid fluid overload. |
| Intrinsic renal | Acute tubular injury, glomerulonephritis, interstitial nephritis, vascular disease | RBC/WBC/granular casts, significant protein, systemic features, poor recovery after perfusion correction | Remove trigger, obtain appropriate serology/ultrasound and seek nephrology early. |
| Post-renal obstruction | Prostate disease, stones, pelvic mass, clots, blocked catheter | Oliguria/anuria, colic or retention; hydronephrosis may be absent early or with dehydration | Bladder scan, catheter assessment and urgent ultrasound/urology decompression. |
Fractional excretion tests
FENa (%) = 100 × (urine Na × plasma creatinine) ÷ (plasma Na × urine creatinine). A value <1% may support pre-renal physiology and >2% may support tubular injury. However, diuretics, CKD, sepsis, contrast, glomerulonephritis and early AKI reduce accuracy.
FEUrea (%) = 100 × (urine urea × plasma creatinine) ÷ (plasma urea × urine creatinine). It may be less affected by diuretics, but is not definitive. Use trends, history, examination, urine sediment and ultrasound rather than any single fractional excretion.
7. Chronic kidney disease (CKD)
CKD means abnormalities of kidney structure or function present for at least 3 months with health implications. It is diagnosed by eGFR <60 mL/min/1.73 m2 or persistent markers of kidney damage (albuminuria, urine sediment abnormality, tubular/electrolyte disorder, histological abnormality, structural abnormality on imaging or previous transplant).
Risk is reported as a G category plus A category (CGA). Confirm persistence, exclude AKI and transient albuminuria, and assess blood pressure, diabetes, cardiovascular risk, anaemia, bone/mineral disease and medication safety.
8. Recognising important renal syndromes
| Syndrome | Core laboratory pattern | Examples/next steps |
|---|---|---|
| Nephritic | Haematuria (often dysmorphic), RBC casts, variable proteinuria, hypertension and falling GFR | Urgent renal assessment; check complements, ANA, ANCA, anti-GBM, hepatitis/HIV tests as indicated. |
| Nephrotic | Heavy proteinuria, low albumin, oedema and hyperlipidaemia | Quantify ACR/PCR, assess thrombosis/infection risk and search for diabetes, systemic disease or drug cause. |
| Tubulointerstitial | Pyuria/WBC casts, modest proteinuria, impaired concentrating ability | Review antibiotics, NSAIDs, PPIs and other drugs; consider eosinophilia/rash/fever and specialist review. |
| Thrombotic microangiopathy | AKI with thrombocytopenia, anaemia, schistocytes, raised LDH and low haptoglobin | Medical emergency; treat as possible TTP/HUS/DIC while confirming cause—do not wait for every result. |
| Obstructive uropathy | Rising creatinine, oliguria/anuria, variable urine sediment | Bladder scan and renal ultrasound; decompression may be kidney-saving. |
9. Renal emergencies: act before the final report
- Hyperkalaemia: obtain ECG and repeat a non-haemolysed sample, but immediately stabilise the myocardium and shift/remove potassium according to local emergency protocol when severe or ECG-toxic.
- Severe metabolic acidosis: blood gas, lactate, ketones and anion gap; treat the cause and involve nephrology/critical care if refractory.
- Fluid overload/pulmonary oedema: oxygen/ventilatory support, urgent diuretic strategy when appropriate, and renal replacement therapy if refractory.
- Uraemic complications: encephalopathy, pericarditis, seizures, severe bleeding tendency or persistent vomiting require urgent specialist management.
- AEIOU dialysis triggers: refractory Acidosis, dangerous Electrolyte disturbance (especially potassium), toxin with dialysability, refractory fluid Overload, or symptomatic Uraemia. The decision is clinical and trend-based, not a single creatinine threshold.
10. Medication and contrast safety
| Situation | Practical action |
|---|---|
| AKI or rapidly changing creatinine | Recalculate medication safety frequently; eGFR may overestimate function. Hold or adjust nephrotoxins and renally cleared drugs with senior/pharmacy advice. |
| NSAIDs | Avoid when possible in dehydration, CKD, heart failure or with ACE inhibitor/ARB plus diuretic (“triple whammy”). |
| ACE inhibitor/ARB | Can cause a predictable creatinine rise; investigate volume depletion, renal artery disease and hyperkalaemia rather than stopping automatically. Recheck after initiation or dose change. |
| Metformin and diabetes medicines | Follow local eGFR and acute-illness/contrast policies; pause medicines when clinically indicated during hypoxia, shock or significant AKI. |
| Iodinated contrast | Estimate risk, optimise volume status, avoid unnecessary nephrotoxins and monitor high-risk patients; do not deny essential imaging without a risk–benefit discussion. |
| Dialysis | Check whether the drug or metabolite is dialysable and whether a post-dialysis dose is required. |
11. A reproducible interpretation algorithm
- Confirm change: compare creatinine/eGFR with baseline and repeat if unexpected.
- Assess danger: ECG, potassium, bicarbonate/pH, oxygenation, fluid overload, urine output, uraemic symptoms.
- Check urine: dipstick, ACR/PCR and microscopy; look specifically for casts and haematuria.
- Assess perfusion and obstruction: vitals, fluid balance, bladder scan and ultrasound when indicated.
- Classify acute versus chronic: previous results, small echogenic kidneys/anaemia-mineral disease and persistence over ≥3 months support CKD.
- Find and reverse the cause: fluids only when indicated, stop or adjust harmful drugs, treat sepsis, relieve obstruction and investigate glomerular disease.
- Trend and communicate: document stage, likely mechanism, repeat time, escalation plan and medication changes.
12. Worked clinical cases
Case 1: pre-renal AKI
A patient with profuse diarrhoea has creatinine 210 micromol/L from a baseline of 80, low blood pressure, low urine output, bland sediment and no obstruction. This meets AKI criteria. The pattern supports pre-renal hypoperfusion, but sepsis and tubular injury must be reassessed if creatinine fails to improve after careful perfusion correction.
Case 2: glomerular emergency
A patient has rising creatinine, hypertension, oedema, dysmorphic RBCs, RBC casts and A3 albuminuria. This is a nephritic pattern, not uncomplicated dehydration. Urgent nephrology input, serology and possible renal biopsy are required; do not delay escalation while waiting for a “kidney function panel.”
Case 3: obstructive AKI
An older man has anuria, suprapubic discomfort and creatinine rising rapidly. A bladder scan shows retention. Catheter assessment and urgent decompression are priorities. A normal early ultrasound would not completely exclude obstruction if clinical suspicion remains high.
Case 4: CKD risk
A person with diabetes has eGFR 68 and ACR 120 mg/g on two samples four months apart. This is G2A2 CKD, not “normal kidneys.” Address blood pressure and diabetes risk, review renally cleared medicines and arrange ongoing monitoring.
13. Quick self-test
- Why can a normal creatinine conceal serious kidney disease in a frail adult?
- Which urine finding most strongly supports glomerulonephritis: hyaline casts, RBC casts or isolated trace protein?
- What are the three KDIGO AKI criteria domains?
- When can an eGFR of 75 represent CKD?
- Name the AEIOU indications for urgent dialysis discussion.
Answers
- Creatinine production is low with low muscle mass, so serum concentration may not rise despite reduced filtration.
- RBC casts (especially with dysmorphic RBCs) indicate glomerular bleeding.
- Rise in creatinine, percentage rise from baseline, and reduced urine output.
- When a persistent marker of kidney damage (for example A2/A3 albuminuria or structural abnormality) is present for at least three months.
- Refractory Acidosis, dangerous Electrolytes, dialysable toxin, refractory fluid Overload and symptomatic Uraemia.
Key take-home points
- Interpret the trend and the patient, not the creatinine number in isolation.
- Pair filtration (eGFR/creatinine) with urine damage markers (ACR, PCR, sediment) and electrolyte/acid–base status.
- AKI is diagnosed by change over time; stage it and look for pre-renal, intrinsic and post-renal causes immediately.
- Persistent eGFR reduction or kidney damage for ≥3 months defines CKD; use CGA risk classification.
- Hyperkalaemia, pulmonary oedema, severe acidosis, uraemic complications and obstruction are time-critical.
References and further reading
- NCBI Bookshelf: Renal Function Tests
- NIDDK: Clinical measurements and eGFR accuracy
- MedlinePlus: Kidney tests
- KDIGO: Acute kidney injury guideline
- KDIGO: CKD evaluation and management
- MedlinePlus: Albumin–creatinine ratio
Educational note: This resource supports learning and emergency recognition. Apply local protocols, senior/renal consultation and the patient’s complete clinical picture when making decisions.
