What does it measure?
The calculation divides BUN by creatinine using the laboratory's specified units. Both depend on kidney handling, while BUN also varies with protein metabolism and creatinine with muscle. A ratio using urea instead of BUN gives a different number and needs a different interpretation. [2]
Before reading the calculation
- Inputs
- BUN divided by creatinine. A urea-to-creatinine ratio uses a different convention and is not numerically interchangeable.
- Original values
- Keep both component values and units. A low creatinine can raise the ratio even without high BUN.
What can the result mean?
A high ratio can occur when BUN rises more than creatinine, such as with reduced blood flow to the kidneys or digestive bleeding. It can also appear when creatinine is simply low. A low ratio can reflect lower urea production or a relatively higher creatinine. [1]
BUN divided by creatinine. A urea-to-creatinine ratio uses a different convention and is not numerically interchangeable.
- Original values
- Keep both component values and units. A low creatinine can raise the ratio even without high BUN.
When is it useful, and how is it read?
When kidney results change, the individual BUN and creatinine values, urine output, fluid history, and examination matter more than the ratio in isolation. Protein breakdown, digestive bleeding, and low muscle mass can alter the calculation without supplying one clear explanation. [1] [5]
What can affect it?
Diet, muscle mass, medicines, illness, and fluid balance influence the components. Research in patients with acute kidney injury found that the ratio alone did not reliably distinguish reduced kidney blood flow from injury within the kidneys. It is a contextual clue, not a diagnosis or hydration score. [1]
A high ratio does not prove dehydration
The ratio can rise because BUN increased, creatinine decreased, or both. Research in acute kidney injury found it could not reliably distinguish reduced kidney blood flow from injury within the kidney, so it cannot settle that diagnosis alone. [1]
Questions you might have
Does a high ratio prove dehydration?
No. The ratio alone did not reliably distinguish causes of acute kidney injury in the cited clinical study. [1]
What comes next?
Review the original values, eGFR, urine findings, symptoms, and the trend. Your clinician can decide what additional assessment is needed. Do not assume that a high ratio means you should drink large amounts of water or that an ordinary ratio rules out kidney problems.
Are both original results abnormal, and does the ratio add useful information in my case?
The reference interval and units on your report are part of understanding your result. Understand units, reference intervals, and treatment targets.
Sources & references
Medical references & original research
- Diagnostic performance of serum blood urea nitrogen to creatinine ratio for distinguishing prerenal from intrinsic acute kidney injury in the emergency department (2017)
- MedlinePlus — Bun Blood Urea Nitrogen
- MedlinePlus — Creatinine Test
Further reading
Books we consulted
References that informed our explanations of the test and its context.
- Pagana KD, Pagana TJ, Pagana TN. Mosby’s Diagnostic and Laboratory Test Reference. 14th ed. Elsevier; 2019.
Practical-section sources accessed: .
For general health education. Interpreting your result and deciding on tests or treatment requires your health context and a qualified clinician.
