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The History of the BUN/Creatinine Ratio

Creatinine was first isolated in 1847 by German chemist Justus von Liebig, who extracted it from concentrated meat extract while studying general muscle chemistry, decades before its important connection to kidney function was ever properly understood by physicians studying the kidneys. Once both creatinine and blood urea nitrogen became routine, separately measured laboratory tests by the mid-20th century, physicians gradually noticed that comparing the two values together, rather than examining either one in complete isolation, revealed diagnostic patterns that neither number could reveal entirely on its own. This observation slowly developed into a standard diagnostic framework taught throughout nephrology training programs everywhere, helping doctors distinguish between genuinely different underlying causes of reduced kidney function using nothing more than simple arithmetic applied to two already-common blood tests routinely ordered together as a pair.

What the BUN/Creatinine Ratio Actually Is

This ratio compares two different waste products that both tend to rise when the kidneys aren't filtering blood efficiently, but for genuinely different underlying physiological reasons rooted in how each substance behaves. Creatinine comes from ordinary, everyday muscle metabolism at a fairly constant and predictable rate, and isn't reabsorbed much by the kidneys once it's filtered out of the blood, while urea production fluctuates considerably with diet and liver function, and importantly, urea can be reabsorbed back into the bloodstream by the kidneys under certain conditions, especially during dehydration. Because of this key difference, a disproportionately high ratio often points toward a "prerenal" problem, such as dehydration or reduced blood flow to the kidneys themselves, whereas a normal ratio with both values elevated together more often suggests direct damage to the kidney tissue itself.

How the BUN/Creatinine Ratio Is Measured

Like the anion gap, this value isn't measured directly at all but instead calculated by dividing a separately measured BUN result by a separately measured creatinine result from the very same blood draw taken that day. BUN is measured using the enzyme urease to convert urea into detectable ammonia, while creatinine is measured using the Jaffe reaction, developed in 1886, in which creatinine reacts with picric acid under alkaline conditions to form an orange-red compound whose intensity a spectrophotometer can precisely quantify. Both underlying tests run automatically on the very same modern chemistry analyzer, making the ratio essentially free to calculate once the two individual results are already available to the physician.

Scientists Behind the Science

Justus von Liebig's 1847 isolation of creatinine from muscle tissue gave scientists their first genuinely pure sample of the compound to study in careful detail, decades before its clinical relevance to kidney function was ever formally recognized by the medical community. German physician Max Jaffe's 1886 color-reaction test, still fundamentally the basis of modern creatinine assays used today, made routine creatinine measurement genuinely practical for the first time in ordinary hospital laboratories everywhere in the world. Combined with Donald Van Slyke's earlier work standardizing blood urea testing, these two separate 19th-century discoveries eventually converged decades later into the single, simple diagnostic ratio nephrologists still rely on every single day in clinical practice around the globe.