How is the SI ratio calculated?

SI labs report urea in mmol/L and creatinine in micromol/L, so a direct ratio is not comparable to the US BUN:creatinine number. The tool converts urea to BUN by multiplying by 2.8 (BUN mg/dL = urea mmol/L × 2.8) and creatinine micromol/L to mg/dL by dividing by 88.4, then takes the ratio on the same conventional basis.

What does a high ratio mean?

A BUN:creatinine ratio above about 20:1 suggests a pre-renal cause such as dehydration, blood loss, heart failure, or gastrointestinal bleeding, where the kidney reabsorbs urea avidly while creatinine rises less. It points to reduced renal perfusion rather than intrinsic kidney damage.

What does a normal or low ratio mean?

A ratio of roughly 10:1 to 20:1 is the normal range and, in the context of a raised creatinine, suggests intrinsic renal disease such as acute tubular necrosis. A low ratio can also occur with low protein intake, liver disease, or over-hydration, which the tool notes as caveats.

Can this diagnose post-renal obstruction?

Not on its own. Post-renal obstruction often raises the ratio like a pre-renal picture early on, then behaves like intrinsic disease as it persists. The tool flags that an elevated ratio with relevant symptoms warrants imaging to exclude obstruction; it cannot confirm it from the numbers alone.

What limits the ratio's reliability?

Urea is affected by protein intake, gastrointestinal bleeding, steroids, catabolism, and liver function, while creatinine depends on muscle mass. High or low urea from these causes shifts the ratio independently of kidney perfusion, so always interpret it alongside the clinical picture and urinary indices.

What is the BUN:Creatinine Ratio Calculator?

Calculates the BUN:creatinine ratio in conventional units, or the urea:creatinine ratio in SI units, and classifies the pattern as pre-renal (above 20:1), intrinsic renal (10 to 20:1), or suggests a post-renal cause. For nephrology and general medicine. It runs free in your browser on Gera Tools, with nothing uploaded.

BUN:Creatinine Ratio Calculator

Name: BUN:Creatinine Ratio Calculator
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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The ratio of blood urea nitrogen to creatinine helps separate the causes of a rising creatinine. This calculator computes the ratio from conventional or SI lab values and classifies the azotaemia as pre-renal, intrinsic renal, or a pattern that should prompt a search for obstruction.

Why the ratio separates causes

Urea and creatinine are both nitrogenous waste products filtered by the kidney, but the kidney handles them differently. Creatinine is filtered and barely reabsorbed — its plasma level rises in proportion to the fall in glomerular filtration rate (GFR) regardless of how much urine the kidney makes. Urea (reported as BUN in the United States), by contrast, is also freely filtered but is reabsorbed passively along the tubule, especially when flow through the collecting duct is slow.

When kidney perfusion falls — dehydration, heart failure, blood loss — the tubules are exposed to slow-moving filtrate for longer, and more urea is reclaimed. Creatinine reabsorption changes little. The result is that BUN rises disproportionately, pushing the ratio above the normal 10:1–20:1 range. This is the physiological basis of the pre-renal pattern.

How it works

The ratio is computed on the conventional BUN:creatinine basis, converting SI values first:

Conventional: ratio = BUN (mg/dL) / creatinine (mg/dL)

SI conversion before ratio:
  BUN (mg/dL)        = urea (mmol/L) × 2.8
  creatinine (mg/dL) = creatinine (micromol/L) / 88.4

Interpretation:
  ratio > 20      -> pre-renal (or early post-renal) — reduced perfusion
  ratio 10 to 20  -> normal range / intrinsic renal if creatinine raised
  ratio < 10      -> low protein, liver disease, or over-hydration

Worked examples

Example 1 — Pre-renal (dehydration). A 70-year-old with vomiting for two days. BUN 65 mg/dL, creatinine 2.0 mg/dL. For example, ratio = 65 ÷ 2.0 = 32.5:1. Interpretation: pre-renal. Fluid resuscitation typically normalises both values quickly.

Example 2 — Intrinsic renal (acute tubular necrosis). After a contrast procedure. BUN 48 mg/dL, creatinine 3.5 mg/dL. For example, ratio = 48 ÷ 3.5 = 13.7:1. Within the 10–20 range, consistent with intrinsic renal disease where tubular reabsorption of urea is impaired alongside GFR.

Example 3 — SI units. Urea 22 mmol/L, creatinine 190 micromol/L. BUN = 22 × 2.8 = 61.6 mg/dL; creatinine = 190 ÷ 88.4 = 2.15 mg/dL. Ratio = 61.6 ÷ 2.15 ≈ 28.7:1 — pre-renal pattern.

Common confounders to consider

The BUN:creatinine ratio moves for reasons other than perfusion:

High ratio despite normal perfusion: gastrointestinal bleeding raises urea load (blood is digested like protein); high-protein diet; steroid use; catabolism.
Low ratio despite impaired kidneys: severe liver disease (reduced urea synthesis), very low protein intake, rhabdomyolysis (creatinine floods in from muscle damage).
Low muscle mass: elderly or cachectic patients may have a deceptively low baseline creatinine, making the ratio appear elevated even when perfusion is adequate.

Always interpret the ratio alongside the clinical picture, urinary indices (sodium, osmolality, fractional excretion of sodium), and the urine sediment. This tool is for educational use and supports clinical assessment — it is not a substitute for physician judgement.