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.

BUN:Creatinine Ratio Calculator

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.

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

The kidney reabsorbs urea more avidly than creatinine when perfusion falls, so pre-renal states push the ratio up.

Example and notes

A BUN of 60 mg/dL with a creatinine of 2.0 mg/dL gives a ratio of 30:1, a pre-renal pattern consistent with dehydration or gastrointestinal bleeding. In SI units, a urea of 21 mmol/L (BUN ≈ 59 mg/dL) with a creatinine of 177 micromol/L (≈ 2.0 mg/dL) gives the same answer. Always interpret the ratio alongside the clinical picture, urinary sodium, and fractional excretion of sodium, since protein intake, bleeding, and muscle mass all move it independently of perfusion.