It is said that a high urea:creatinine ratio is an indicator of a pre-renal cause of acute kidney injury [1]. This is usually discussed in United States units, where urea is measured as blood urea nitrogen (BUN) in mg/dL, and creatinine in mg/dL. The cut-off for a high BUN:creatinine ratio is usually given as >20 (mg/mg) [1–3].

Outside of the United States, urea is usually expressed in SI units mmol/L, and creatinine in μmol/L. The cut-off for a high urea:creatinine ratio is usually given in SI units as >100 (mmol/mmol) [1]. However, these cut-offs are not equivalent.

The molar mass of (molecular) nitrogen is 28.014 g/mol [4], so a BUN of 1 mg/dL is equivalent to a urea of $\frac{1\ \text{mg/dL}}{28.014\ \text{g/mol}} = \frac{10\ \text{mg/L}}{28.014\ \text{mg/mmol}}$ = 0.357 mmol/L.

Similarly, the molar mass of creatinine is 113.12 g/mol, so creatinine of 1 mg/dL is equivalent to $\frac{1\ \text{mg/dL}}{113.12\ \text{g/mol}} = \frac{10\ \text{mg/L}}{113.12\ \text{mg/mmol}}$ = 0.0884 mmol/L.

So a BUN:creatinine ratio of 20 (mg/mg) is equivalent to an SI urea:creatinine ratio of 20 × $\frac{0.357}{0.0884}$ = 80.8 ≈ 80 (mmol/mmol), not 100.

(Note, however, that the evidence for BUN:creatinine or urea:creatinine ratio in distinguishing pre-renal kidney injury is limited [2–3].)

References

[1] Nickson C. Urea-creatinine ratio. Life in the Fast Lane; 2020 Nov 3 [cited 2022 Sep 20]. https://litfl.com/urea-creatinine-ratio/

[2] Manoeuvrier G, Bach-Ngohou K, Batard E, et al. Diagnostic performance of serum blood urea nitrogen to creatinine ratio for distinguishing prerenal from intrinsic acute kidney injury in the emergency department. BMC Nephrology. 2017 May 25; 18(1): Article 173. doi: 10.1186/s12882-017-0591-9

[3] Uchino S, Bellomo R, Goldsmith D. The meaning of the blood urea nitrogen/creatinine ratio in acute kidney injury. Clinical Kidney Journal. 2012 Apr 1; 5(2): 187–91. doi: 10.1093/ckj/sfs013

[4] National Center for Biotechnology Information. PubChem. Bethesda (MD): National Library of Medicine; [cited 2022 Sep 20]. https://pubchem.ncbi.nlm.nih.gov/