The factor of 1000 is needed to convert the creatinine result from μmol/L to mmol/L, the urea unit of measurement. The SI ratio (UCR) is plasma urea (mmol/L) / (plasma creatinine (μmol/L) divided by 1000). The reference range is around 8-15 and the most commonly used cut-off value to define increased BCR is 20. The non-SI ratio (BCR) is BUN (mg/dL) / plasma creatinine (mg/dL). These two different ways of reporting urea results give rise to two quite different values for the ratio. In all other parts of the world, urea results are expressed as the whole urea molecule (MW 60) and reported as urea in SI units (mmol/L). In the US and a few other countries, urea concentration is expressed as the nitrogen content of urea (MW 28) and reported as blood urea nitrogen (BUN) in non-SI units (mg/dL). The distinction between BUN: creatinine ratio (BCR) and urea: creatinine ratio (UCR) In most cases of chronic renal disease, the ratio remains relatively normal. When there is decreased formation of urea, as seen in liver disease, there is a decrease in the BUN/Creatinine ratio. Increased urea formation also results in an increase in the ratio, e.g. congestive heart failure or recent onset of urinary tract obstruction, will result in an increase in the BUN/Creatinine ratio.

The BUN/Creatinine ratio is useful in the differential diagnosis of acute or chronic renal disease. The ratio may be used to determine the cause of acute kidney injury or dehydration. The units of creatinine are also different (μmol/L), and this value is termed the urea-to-creatinine ratio. In Canada and Europe, the units are also different (mmol/L).

Urea nitrogen bun normal range