BUN/Creatinine Ratio Overview
The BUN/Creatinine Ratio is a useful indicator for assessing kidney function. It consists of two separate tests which include Blood Urea Nitrogen (BUN) and Creatinine, which are normally ran on renal function panels, basic metabolic panels and comprehensive metabolic panels. Individually, each result gives helpful information on the health and function of the kidneys, but with the BUN/Creatinine Ratio, this value gives a single number which can give medical providers an alternate view of a quick glimpse at kidney health.
What is the BUN/Creatinine Ratio Formula?
-Formula: BUN / Creatinine
-Example: 15mg/dL / 1.1mg/dL= 13.63 ratio
Preparing For The Test
Some providers may require fasting, especially if testing in conjunction with other lab panels that require it. Either serum or plasma blood samples will be required. For labs requiring plasma, a green top tube with lithium heparin anticoagulant will be drawn. For those preferring serum, a gold top SST (Serum Separator Tube) will be drawn.
If the specimen gets to the lab and it is hemolyzed, this is grounds for being rejected due to potential for erroneous results. If a specimen is rejected, the patient will be recalled for collection of a new specimen. Proper handling by the medical staff is crucial in preventing specimens from rejection.
Although the BUN and Creatinine tests can be run individually or as part of a Kidney Function Panel, many labs have a specific order available which will specify as “BUN/Creatinine Ratio”.
When Is The Best Time Of Day To Test The BUN/Creatinine Ratio?
Due to the significant effect of diurnal variation with this test, a doctor needs to make that call. Test results can fluctuate depending what time of day the sample is taken. This is called diurnal variation and affects some analytes more than others.
One recent study out of Turkey analyzed 19 analytes in 17 test subjects and accumulated this data after drawing baseline blood samples at 9.00 am along with 4 additional draws throughout a 24-hour period (Alperen Halil Ihtiyar, 2023). The results concluded that there was minimal change in Creatinine levels at different times of day. However, there were very significant variations in BUN results at 12.00 am, 3.00 pm, 6.00 pm and 12.00 pm. These results varied 10.09%, 12.58%, 12.31%, and 25.02% higher than the baseline results, respectively.
As seen in the above data, the time of day can have a drastic effect on BUN results, up to an average of 25% higher if sampled at 12.00 pm.
Let’s look at an example to see how this would impact a patient’s BUN/Creatinine ratio.
Drawn at Baseline (9.00 am): 16.0 / 0.9= Ratio of 16
Drawn at 12.00 pm: 20.0 / 0.9= Ratio of 20
We can see that as expected, the Creatinine didn’t move. But in the 12.00 pm sample, the BUN was 25% higher than the 9.00 am sample (4 mg/dL) higher. When calculated, this moved the BUN/Creatinine Ratio from within normal range to being on the high side. This gives the provider a false elevated result and could potentially affect treatment and the clinical outcome.
How Long For BUN/Creatinine Ratio Results
Results for this ratio are often received back the same day. BUN and Creatinine are both very common analytes which are ran in high volumes every day in most laboratories. It does not typically require ship out to a higher echelon laboratory, but is usually run in-house on an automated immunoassay analyzer.
Interpreting Results
- Normal BUN values for an adult are between 8-21 mg/dL.
- Normal Creatinine values are between 0.61-1.21 mg/dL for adult males and between 0.51-1.11 mg/dL for adult females. The difference in creatinine range is due to the higher average muscle mass of male patients in comparison to females, considering that this is a product of muscle catabolism.
- Although both of these are classic markers of kidney function, many more metabolic factors can cause a shift in BUN results than Creatinine. This is why Creatinine is widely considered a much more specific indicator of renal health.
- When either one, or both of these results is out of range, it can cause the ratio to be higher or lower than normal. The normal ratio of BUN to Creatinine should be between 10:1 to 20:1.
- One thing that the BUN/Creatinine Ratio is supposed to identify is the nature of Azotemia (buildup of nitrogenous products in the blood) a patient is encountering and whether it is pre-renal, renal or post-renal. However, numerous studies have been conducted which concluded that the ratio was not as helpful as previously thought. Hence, one can best think of this as a rough guide, rather than a clear-cut measure.
High BUN/Creatinine Ratio
- An increased ratio may be due to conditions that cause a decreased blood flow to the kidneys, such as congestive heart failure or dehydration.
- High protein diets can also cause increased levels, since the additional protein consumed produces extra urea.
- GI bleeding can cause high levels. The American Journal of Gastroenterology published a study in an attempt to determine a correlation between upper gastrointestinal bleeding (UGIB) and high BUN/Creatinine ratios. According to this study, 1,194 patients that were tested had a specificity of 85% for UGIB when they were at a ratio of 30:1 (Patel, et al., 2019). This likelihood may be further increased when there is a decrease in hemoglobin levels, a common tell-tale sign of hemorrhaging.
Low BUN/Creatinine Ratio
A low ratio can be seen with malnutrition/inadequate protein intake, advanced liver disease, sickle cell anemia, rhabdomyolysis and overhydration.
Results Within Normal Range
- Even though results may be within the normal range of 10:1 to 20:1, does not necessarily mean that there are no issues. Since this is a ratio, the result is going to be dependent on the relation of the numerator and denominator together. If the numerator (BUN) and denominator (Creatinine) are both high, you can still have an in-range result. Likewise, even if both results are low, your ratio can still come back normal. This can be illustrated with these 2 examples below:
–Example(When BUN & Creatinine are both high)
30 (BUN) / 3 (Creatinine)= Ratio of 10
–Example(When BUN & Creatinine are both low)
5 (BUN) / 0.5 (Creatinine)= Ratio of 10
- The times you are most likely to see a high or low ratio is when one result is normal and the other is either high or low. See the example below for a scenario that would cause a low result:
–Example(When BUN is normal & Creatinine is high)
15 (BUN) / 3 (Creatinine)= Ratio of 5
- Now, lets look at a scenario that could cause a high result:
–Example(When BUN is high & Creatinine is normal)
30 (BUN) / 1 (Creatinine)= Ratio of 30
These examples show how potentially critical results may be masked, due to the mere nature of the formula.
Conclusion
The BUN and Creatinine tests are valuable by themselves, but when combined together, make a useful guide for several conditions including renal function and health. However, due to the fact of many non-renal factors that can affect this ratio, in addition to the variability of the analyte values in the formula, it should not be relied upon by itself for a status of any condition.
References
Alperen Halil Ihtiyar, M. H. (2023). The Effect Of Diurnal Variation On Laboratory Tests. Journal Of Basic & Clinical Health Sciences, 387-395. doi:https://doi.org/10.30621/jbachs.1122518
Hosten, A. O. (1990). Chapter 19- 3BUN and Creatinine. In M. W. H Kenneth Walker, Clinical Methods: The History, Physical, and Laboratory Examinations. 3rd edition. (pp. 874-878). Retrieved from https://www.ncbi.nlm.nih.gov/books/NBK305/pdf/Bookshelf_NBK305.pdf
Leeuwen, V., & Bladh. (2019). Davis’s Comprehensive Manual of Laboratory & Diagnostic Tests with Nursing Implications. In V. Leeuwen, & Bladh, Davis’s Comprehensive Manual of Laboratory & Diagnostic Tests with Nursing Implications (p. 373).
Patel, S. M., Peraza, J. M., Hasani, A. M., Luther, S. M., Chugh, R. M., & Tokayer, A. M. (2019, October). Finding the Ideal BUN to Creatinine Ratio in an Upper GI Bleed. The American Journal of Gastroenterology, S355. DOI: 10.14309/01.ajg.0000591980.77707.20
