Free · Laboratory measurement ratios

BUN to Creatinine Mass Ratio

Compare blood urea nitrogen with serum creatinine after converting both observations to mg/dL.

Formula & worked example
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How to use this calculator

Enter the known values in the units shown. Results update as you type.
Where results are editable, change one to solve backwards. Lock a value to hold it fixed.
Use the worked example to check the method. Reset restores the starting fields.

Use the result with context

Health results are educational estimates, not a diagnosis or treatment recommendation. A qualified clinician can interpret measurements in the context of your health.

Formula and method

BUN 20 mg/dL and creatinine 1 mg/dL give a ratio of 20.

Mass ratio = BUN (mg/dL) / creatinine (mg/dL).

Worked example

Enter these known values and leave the other values blank.

Blood urea nitrogen (BUN): 20 mg/dL
Serum creatinine: 1 mg/dL

BUN/creatinine mass ratio: 20

Assumptions and limitations

BUN mass represents nitrogen within urea, not whole-molecule urea mass. The CDC NHANES convention converts BUN mg/dL to mmol/L by multiplying by 0.357; creatinine mg/dL converts to µmol/L by multiplying by 88.4. The ratio always divides mass concentrations in mg/dL.
Primary adult acute-kidney-injury studies found inadequate discriminatory performance for prerenal versus intrinsic injury. Neither the historical cutoff of 20 nor the source’s lower cutoff of 10 establishes diagnosis; do not infer normal kidney function from an intermediate result.
The output is a one-way ratio, not an exposed inverse. Positive concentrations are required; use matching clinical measurements and appropriate laboratory interpretation.

Common questions

Is BUN the same mass concentration as urea?

No. BUN measures nitrogen within urea. Whole-molecule urea has a different mass-to-molar conversion; do not substitute one mass concentration for the other.

Does a ratio above 20 prove dehydration or prerenal kidney injury?

No. Primary diagnostic-performance studies do not support using this ratio alone to distinguish causes.

References

Use the CDC NHANES BUN mass-to-molar conversion convention 0.357 mmol/L per mg/dL in place of the coarser captured 0.36, while retaining creatinine conversion 88.4 µmol/L per mg/dL. The ratio uses BUN and creatinine in mg/dL.

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