Bedside Snapshot
- Normal (Adult)
- 10–20:1 (BUN mg/dL ÷ creatinine mg/dL)
SI ≈40–80 (urea ÷ creatinine, both mmol/L) - Critical Values
- No universal critical value: interpret by trend and clinical context. Act on creatinine, K⁺, pH and hemodynamics.
- High Suggests
- Upper GI bleeding (often >30:1)
- Hypovolemia, shock, decompensated heart failure
- Catabolism, steroids, protein feeds
- Low muscle mass (older, frail)
- Low Suggests
- Liver failure
- Rhabdomyolysis
- Malnutrition
- SIADH, osmotic diuresis, recent dialysis
- Act Now If
- Ratio >30:1 with melena, hematemesis or tachycardia: manage as upper GI bleed; IV access, type and crossmatch. Creatinine up ≥0.3 mg/dL or oliguria: treat as AKI; check K⁺.
- Watch Out For
- A normal ratio can hide severe AKI: BUN 60 and creatinine 4 mg/dL give 15:1. Always read the absolute values and the trend.
Reference & Critical Ranges
Lab-specific ranges: Every lab sets its own interval, by analyzer and by population. Trust the range printed on your report. The values below are typical only.
Reference ranges
| Population | Conventional | SI | Notes |
|---|---|---|---|
| Adult | 10–20:1 | ≈40–80 | Typical teaching interval. Calculated from blood urea nitrogen (BUN) and creatinine on the same sample; many labs do not report the ratio. |
| Older adult | Often >20:1 | Often >80 | Low muscle mass lowers creatinine, so the ratio rises without volume depletion. |
| Pregnancy | No validated interval | — | BUN and creatinine both fall as glomerular filtration rate (GFR) rises; judge each against pregnancy norms, not the ratio. |
| Children | No validated interval | — | Low creatinine (small muscle mass) makes ratios above 20 common in well children. Use age-specific BUN and creatinine ranges. |
Severity & critical values
| Level | Conventional | SI | What it means |
|---|---|---|---|
| Low | <10:1 | <40 | Low urea production (liver failure, malnutrition), urea loss (syndrome of inappropriate antidiuretic hormone [SIADH], osmotic diuresis, dialysis) or high creatinine (rhabdomyolysis, assay interference). |
| Typical | 10–20:1 | ≈40–80 | Expected pattern. Does not exclude acute kidney injury (AKI) when BUN and creatinine are both high. |
| Elevated | >20:1 | >80 | Traditional prerenal cutoff; also GI bleeding, catabolism, steroids, low muscle mass. Does not reliably separate prerenal from intrinsic AKI. |
| Marked | >30:1 | >120 | In a patient with GI bleeding, favors an upper source (specific but insensitive). |
| Critical | None | None | No panic value exists for the ratio. Act on the creatinine trend (KDIGO stage), K⁺, acid–base status, hemoglobin and hemodynamics. |
Critical values: The ratio has no panic value. Escalate on its parts and on the patient. Watch for a rising creatinine or oliguria (AKI), hyperkalemia, acidosis, a falling hemoglobin or shock.
Conversion: Multiply BUN mg/dL by 0.357 to get urea mmol/L. Multiply creatinine mg/dL by 88.4 to get µmol/L. The SI ratio (urea mmol/L ÷ creatinine mmol/L) comes to the conventional ratio × 4, so 20:1 is roughly 81. If creatinine stays in µmol/L, that same division gives 0.081. UK-style SI reports give urea (mmol/L) ÷ creatinine (µmol/L) × 1000, which lands on the same scale as the mmol/L ratio above. Some labs use >100 as the prerenal line. That is about 25:1 conventional, a higher bar than the >80 (about 20:1) this page uses.
BUN:creatinine ratio
BUN (mg/dL) ÷ creatinine (mg/dL)
Typical adult value: 10–20:1.
Causes of Abnormal Results
High (elevated ratio >20:1)
Elevated means >20:1. Marked means >30:1. Before you blame dehydration, ask what is feeding urea production.
| Mechanism | Common causes | Bedside clue |
|---|---|---|
| Protein load in the gut | Upper GI bleeding (digested blood is absorbed as protein); high-protein enteral or parenteral feeds | Melena, hematemesis, coffee-ground emesis, tachycardia, falling hemoglobin; ratio often >30:1 |
| Low effective renal perfusion (more urea reabsorbed) | Hemorrhage, hypovolemia, sepsis, cardiogenic shock, vomiting, diarrhea, overdiuresis, NSAIDs and ACE inhibitors or ARBs (blunt renal perfusion) | Tachycardia, hypotension, poor capillary refill, low urine output |
| Neurohormonal activation in heart failure | Decompensated heart failure, cardiorenal syndrome | Edema, crackles, raised jugular venous pressure; a high ratio here marks higher mortality, not a need for fluid |
| Catabolism (more urea made) | Fever, infection, trauma, thyrotoxicosis; corticosteroids; tetracyclines | Fever, injury, new steroids |
| Low creatinine generation | Older age, frailty, sarcopenia, amputation, spinal cord injury | Low muscle mass; a ‘normal’ creatinine may hide a low GFR |
Do not anchor on dehydration: Melena, hematemesis or a falling hemoglobin with a high ratio means GI bleed until you prove otherwise. In heart failure, a reflex fluid bolus can make congestion worse.
Low (ratio <10:1)
| Mechanism | Common causes | Bedside clue |
|---|---|---|
| Low urea synthesis | Acute liver failure, advanced cirrhosis | Jaundice, encephalopathy, coagulopathy, hypoglycemia, high ammonia |
| Low protein intake | Malnutrition, very low-protein diet | Cachexia, low albumin |
| Extra urea loss or dilution | SIADH and other volume-expanded states, osmotic diuresis (glucose, mannitol), recent dialysis | Euvolemic hyponatremia, glycosuria, recent dialysis |
| Creatinine high relative to BUN | Rhabdomyolysis (muscle releases creatinine), high muscle mass, creatine supplements | Crush injury, prolonged down time, seizures, dark urine; check creatine kinase (CK) |
| Creatinine falsely high | Drugs that block tubular creatinine secretion (trimethoprim, cimetidine); Jaffe-assay interference (ketoacids in diabetic ketoacidosis [DKA], some cephalosporins) | New trimethoprim-sulfamethoxazole; DKA with an unexpectedly high creatinine |
Low ratio plus confusion or jaundice: Consider liver failure. Check glucose, international normalized ratio (INR) and ammonia. After a crush injury or long lie, a low ratio with dark urine points to rhabdomyolysis. Check CK and K⁺.
Kidney injury causes that change the ratio or creatinine
| Type | Examples | Ratio & bedside clue |
|---|---|---|
| Postrenal obstruction | Stones, prostate enlargement, bladder outlet obstruction | Ratio is variable and often normal early; scan the bladder early and catheterize retention |
| Hemodynamic creatinine rise (drug) | NSAIDs and ACE inhibitors or ARBs (reduced glomerular perfusion); SGLT2 inhibitors (early eGFR dip) | NSAIDs and ACE inhibitors or ARBs can raise the ratio like other low-perfusion states; an early SGLT2 inhibitor creatinine rise is an expected hemodynamic effect, not a reason to stop without other signs |
| Intrinsic tubular injury | Contrast, aminoglycosides, sepsis, ischemia (acute tubular necrosis, ATN) | The ratio does not reliably separate ATN from prerenal injury; urine microscopy and urine indices help |
| Recovery (post-ATN diuresis) | Polyuric phase after ATN | Urea is cleared faster than creatinine, so the ratio can fall low while creatinine is still high; judge by trend and output |
Interpretation & Bedside Approach
Step-by-step approach
- Is it real? Use BUN and creatinine from the same sample. If ketoacidosis, jaundice or a cephalosporin could interfere, ask whether creatinine was run by a Jaffe or enzymatic method.
- How sick is the patient? Perfusion, mental status, urine output, K⁺ and pH come first; the ratio never outranks them.
- Read the absolute values and the trend: Compare creatinine with the patient's baseline. A 15:1 ratio with BUN 60 mg/dL and creatinine 4 mg/dL is severe kidney injury.
- Stage the injury: Apply KDIGO 2012 AKI criteria (table below) to creatinine and hourly urine output.
- Find the cause: Sort prerenal, intrinsic and postrenal causes with the history, volume exam, bladder scan or ultrasound, urine microscopy and urine indices, never the ratio alone.
- Ask the GI-bleed question: A ratio >30:1 with melena or hematemesis makes an upper source more likely. A lower ratio does not rule one out.
- Reassess after treatment: Creatinine returning toward baseline within days of restoring perfusion supports a functional (prerenal) cause; a persistent rise suggests structural injury.
KDIGO 2012 AKI definition and staging
AKI: creatinine rise ≥0.3 mg/dL (≥26.5 µmol/L) in 48 h, or to ≥1.5 × baseline within 7 days, or urine output <0.5 mL/kg/hr for 6 h.
| Stage | Serum creatinine | Urine output |
|---|---|---|
| 1 | 1.5–1.9 × baseline, or rise ≥0.3 mg/dL (≥26.5 µmol/L) | <0.5 mL/kg/hr for 6–12 h |
| 2 | 2.0–2.9 × baseline | <0.5 mL/kg/hr for ≥12 h |
| 3 | 3.0 × baseline, or ≥4 mg/dL (≥353.6 µmol/L), or start of kidney replacement therapy, or estimated GFR (eGFR) <35 mL/min/1.73 m² in patients under 18 | <0.3 mL/kg/hr for ≥24 h, or anuria for ≥12 h |
Tests that sort the cause better than the ratio
| Finding | Suggests | Caveat |
|---|---|---|
| Fractional excretion of sodium (FENa) <1% | Prerenal | Most accurate in oliguric patients without chronic kidney disease (CKD) or diuretics; unreliable on diuretics |
| FENa >2% (urine Na usually >40 mmol/L) | ATN (intrinsic) | Usual ATN-leaning range; FENa 1–2% is indeterminate; cutoffs vary by population |
| Urine osmolality >500 mOsm/kg | Prerenal | Intact concentrating ability; below about 350 mOsm/kg favors ATN; unreliable on diuretics |
| Fractional excretion of urea (FEUrea) ≤35% | Prerenal | Useful when the patient has had diuretics; only moderate accuracy, cutoffs range 30–40% |
| Muddy brown granular casts, renal tubular epithelial cells | Acute tubular necrosis (ATN) | Needs fresh urine |
| Hydronephrosis or a large bladder volume | Postrenal obstruction | Scan the bladder early; catheterize urinary retention |
| Ratio >30:1 with melena or hematemesis | Upper GI bleeding | Specific but insensitive |
Fractional excretion of sodium (FENa)
(urine Na × plasma creatinine) ÷ (plasma Na × urine creatinine) × 100
The ratio does not diagnose prerenal AKI: Large emergency department and hospital cohorts found it no better than chance at telling prerenal from intrinsic AKI. A ratio >20:1 tracked with higher mortality, not lower. Trust the trend over any single value.
Special Populations
| Population | What changes | Clinical implication |
|---|---|---|
| Neonates & infants | Creatinine at birth reflects the mother's and falls over the first weeks; no standardized ratio exists | Use age-specific BUN and creatinine ranges and the trend; do not apply adult ratio cutoffs |
| Children & adolescents | Small muscle mass keeps creatinine low, so the ratio often exceeds 20:1 in well children | Assess dehydration clinically (capillary refill, skin turgor, breathing pattern); see pediatric vital signs |
| Pregnancy | GFR rises; BUN and creatinine both fall | A creatinine in the non-pregnant ‘normal’ range may already mean reduced function; the ratio adds little |
| Older adults | Lower muscle mass lowers creatinine and raises the ratio | A high ratio with a ‘normal’ creatinine can hide a low GFR; follow the trend and urine output |
| CKD / dialysis | Both values are chronically high; dialysis removes urea and creatinine | Compare with the patient's baseline and dialysis timing; the ratio cannot type AKI on dialysis |
| Liver disease | Low urea synthesis lowers BUN; muscle wasting lowers creatinine | Low BUN and creatinine overestimate kidney function and can mask significant AKI, including hepatorenal syndrome |
| Other: high muscle mass, amputation, spinal cord injury | Creatinine generation is high (athletes, creatine use) or low (amputation, paralysis) | Ratio shifts without any change in kidney function; cystatin C-based eGFR helps |
Interfering Factors & Pitfalls
Collection & handling
| Factor | Effect | Why | Avoid / fix |
|---|---|---|---|
| Unseparated sample (Jaffe creatinine) | Creatinine ↑ → ratio ↓ | Measured creatinine creeps up the longer serum stays on the cells | Separate serum or plasma from cells promptly (per your lab's policy) |
| Hemolysis or icterus | Creatinine bias (direction varies) | Spectral interference, worse with Jaffe | Redraw; ask for an enzymatic creatinine |
Medications & supplements
| Factor | Effect | Why | Avoid / fix |
|---|---|---|---|
| Trimethoprim, cimetidine | Creatinine ↑ → ratio ↓ | Block tubular creatinine secretion without changing GFR | Check the start date; BUN and cystatin C are unaffected |
| Corticosteroids, tetracyclines | BUN ↑ → ratio ↑ | Increase protein catabolism and urea production | Interpret with the drug history |
| Mannitol (osmotic diuresis) | BUN ↓ → ratio ↓ | Osmotic water and urea loss, most often in a volume-expanded patient | Do not read a low ratio as ‘not dry’; check volume status |
| Loop or thiazide diuretics | BUN ↑ → ratio ↑ | Volume depletion and increased proximal reabsorption | Assess volume status; a high ratio on diuretics is often contraction |
Physiologic / biologic
| Factor | Effect | Why | Avoid / fix |
|---|---|---|---|
| Ketoacids (DKA), marked hyperglycemia | Creatinine False + → ratio ↓ | Acetoacetate and glucose react as non-creatinine chromogens in the Jaffe method | Repeat with an enzymatic assay or after treatment |
| Low muscle mass | Creatinine ↓ → ratio ↑ | Less creatinine generated | Use the trend; consider cystatin C |
Assay & device-specific
| Factor | Effect | Why | Avoid / fix |
|---|---|---|---|
| Point-of-care vs central-lab creatinine | Creatinine bias | Devices and methods (Jaffe vs enzymatic) differ in accuracy and interferences | Trend on one platform; confirm key decisions on a lab sample |
Common misinterpretations
- ‘High ratio means give fluid’: No. Not in heart failure, pulmonary edema or GI bleeding. GI bleeding needs blood and source control.
- ‘Low ratio means intrinsic kidney disease’: Usually, a low ratio means little urea was made or creatinine release is high.
- ‘The ratio separates prerenal from ATN’: It doesn't. Lean on the clinical picture, the urine studies and how the patient responds to treatment.
Physiology & Test Basics
What it measures
This page is about one calculated number: the BUN:creatinine ratio. Each component has its own page: BUN (urea production and clearance) and creatinine (the filtration marker). Staging lives on eGFR. The ratio tells you whether BUN has drifted out of step with creatinine.
Physiology & regulation
- Urea: The liver makes it from ammonia released when protein breaks down. The kidney filters it freely, then reabsorbs part of it.
- Creatinine: Muscle makes it from creatine, at a fairly steady pace set by muscle mass. The kidney filters it, and the tubules also secrete it.
- Low perfusion: When perfusion is low, vasopressin and slow tubular flow pull back more water and urea. BUN therefore climbs faster than creatinine.
- Urea production: Production climbs with GI bleeding, protein feeding, catabolism and steroids. It drops with liver failure and malnutrition.
How it is measured
- Specimen: serum (gold or red top) or lithium-heparin plasma (green top), the same tube as the chemistry panel. No fasting needed.
- Calculation: BUN ÷ creatinine (both mg/dL) from one sample; calculate it if the report does not.
- Methods: BUN by urease-based assay; creatinine by Jaffe (colorimetric) or enzymatic assay.
- Point of care: some ED and transport cartridge analyzers report BUN and creatinine from whole blood.
Why it matters
Get BUN and creatinine back together and the ratio is a free clue. It points toward GI bleeding, low perfusion, catabolism, liver failure or rhabdomyolysis. It is weak at sorting out AKI. Use it to raise questions, not to make fluid decisions.
Clinical Pearls
A clue, not a diagnosis: The ratio shows which way BUN and creatinine moved. It doesn't show why. Put it next to the history, the exam, urine output and urine studies.
GI bleeding: Melena or hematemesis with a ratio >30:1 points to an upper source. That argues for early upper endoscopy. A lower ratio does not rule out an upper bleed.
Normal ratio, sick kidneys: BUN 60 mg/dL with creatinine 4 mg/dL gives a ‘normal’ 15:1 ratio, and that is severe AKI. Read the absolute values every time. Compare creatinine with baseline.
Transport handoff (Transport/CCT): Before an interfacility transfer, write down BUN, creatinine, K⁺ and the baseline creatinine. Stamp each value with its draw time. Add urine output since admission. Trend point-of-care results on the same device.
Nursing: urine output is the earliest KDIGO signal: Keep strict intake and output. Chart hourly urine output in mL/kg/hr. Report output <0.5 mL/kg/hr for 6 h, even when creatinine hasn't risen yet.
Heart failure with a high ratio: In decompensated heart failure, a high ratio signals neurohormonal activation. It also tracks with higher all-cause mortality. It is not a fluid deficit. Check for congestion before any bolus.
References
- Kidney Disease: Improving Global Outcomes (KDIGO) Acute Kidney Injury Work Group. (2012). KDIGO Clinical Practice Guideline for Acute Kidney Injury. Kidney International Supplements, 2(1), 1–138. Current guideline; a 2026 update is in draft. https://doi.org/10.1038/kisup.2012.1
- Kidney Disease: Improving Global Outcomes (KDIGO) CKD Work Group. (2024). KDIGO 2024 Clinical Practice Guideline for the Evaluation and Management of Chronic Kidney Disease. Kidney International, 105(4S), S117–S314. https://doi.org/10.1016/j.kint.2023.10.018
- Manoeuvrier, G., Bach-Ngohou, K., Batard, E., et al. (2017). Diagnostic performance of serum blood urea nitrogen to creatinine ratio for distinguishing prerenal from intrinsic acute kidney injury in the emergency department. BMC Nephrology, 18, 173. https://doi.org/10.1186/s12882-017-0591-9
- Uchino, S., Bellomo, R., & Goldsmith, D. (2012). The meaning of the blood urea nitrogen/creatinine ratio in acute kidney injury. Clinical Kidney Journal, 5(2), 187–191. (landmark cohort) https://doi.org/10.1093/ckj/sfs013
- Simadibrata, D. M., Koo, T. H., Murlidhar, M., et al. (2026). Blood urea nitrogen-to-creatinine ratio to differentiate upper from lower gastrointestinal bleeding: A systematic review and meta-analysis. Journal of Gastroenterology and Hepatology, 41(2), 477–487. https://doi.org/10.1111/jgh.70224
- Abdelhafez, M., Nayfeh, T., Atieh, A., et al. (2022). Diagnostic performance of fractional excretion of sodium for the differential diagnosis of acute kidney injury: A systematic review and meta-analysis. Clinical Journal of the American Society of Nephrology, 17(6), 785–797. https://doi.org/10.2215/CJN.14561121
- Pan, H. C., Jiang, Z. H., Chen, H. Y., et al. (2026). Assessing the utility of fractional excretion of urea in distinguishing intrinsic and prerenal acute kidney injury in hospitalised patients: A systematic review and meta-analysis. BMJ Open, 16(1), e100875. https://doi.org/10.1136/bmjopen-2025-100875
- Zhou, Y., Zhao, Q., Liu, Z., & Gao, W. (2024). Blood urea nitrogen/creatinine ratio in heart failure: Systematic review and meta-analysis. PLoS One, 19(5), e0303870. https://doi.org/10.1371/journal.pone.0303870
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