What does the research show about blood urea nitrogen (bun)?

Blood urea nitrogen (BUN)

BUN measures the nitrogen in urea, the waste product of protein breakdown that the kidneys clear. Laboratory sources print reference ranges of 8 to 20 and 6 to 23 mg/dL. Higher values go with more deaths in heart failure and in one general population cohort, and with new diabetes in another. Creatinine-based kidney measures do not always move with it.

Checked 2026-11-13

Status: Part of routine metabolic blood panels, ordered by a clinician and also sold direct to consumers by some US laboratories. FDA classifies urea nitrogen test systems as class II devices [1].
Measures: The nitrogen carried in urea in the blood. Urea is the final product of protein and amino acid breakdown, made in the liver and removed by the kidneys [1][2].
Also called: BUN, blood urea nitrogen, urea nitrogen, serum urea nitrogen, SUN, urea nitrogen blood, urea, serum urea
Tested for: kidney function, dehydration, kidney disease screening, protein intake, heart failure monitoring

Strong several reviews agree · Moderate one review or consistent trials · Limited one or a few small studies · Untested no study in people, listed with what it rests on

Linked in research5

ModerateLinked to more deaths in people hospitalized with heart failure, about 2.3 times the risk in the top quarter of BUN in one cohort.
ModerateLinked to more deaths in hospital patients: a BUN of 20 mg/dL or above went with 4.6 times the odds of death in acute pancreatitis.
LimitedOne US cohort found 48% more deaths from any cause in the highest quarter of BUN than in the lowest over about 12 years.
LimitedOne cohort of 1.3 million US veterans found 23% more new diabetes with BUN above 25 mg/dL than at 25 or below.
LimitedOne study of 459 people with stage 3 to 5 kidney disease found more kidney failure or death in the top quarter of BUN, independent of eGFR.

Reported risks0

No risks sourced yet.

Reference ranges

Each range is the one its source published, in mg/dL. Also reported in mmol/L. Where sources differ, each is shown.

American Board of Internal Medicine laboratory reference ranges, 2026

Adults, as the range printed for board examinations · [3]

Range	What the source calls it
below 8 mg/dL	No name given
8 to 20 mg/dL	"reference range"
above 20 mg/dL	No name given

Published as a reference table for examination candidates. The table does not say how the range was set [3]. Outside the US the same test is reported as urea in mmol/L. A US heart failure registry prints 43 mg/dL as 15.35 mmol/L, which matches the conversion factor above [4].

US national survey laboratory procedure manual, 2017 to 2018

Adults aged 18 to 60, as the reference range printed for the assay · [2]

Range	What the source calls it
below 6 mg/dL	No name given
6 to 23 mg/dL	"reference range"
above 23 mg/dL	No name given

The reference range listed in the laboratory manual for the method used in the 2017 to 2018 US National Health and Nutrition Examination Survey (Roche Cobas 6000). It is the range attached to that assay, not one derived from survey participants, and the manual gives no range for people older than 60 [2].

What has been shown to move it

Shown to lower it2

KetoanaloguesStrong

Phosphate and parathyroid hormone

Shown to lower blood phosphate, urea and parathyroid hormone in adults with chronic kidney disease on a protein-restricted diet.

The numbers and sources

Effect: Ketoanalogues added to a protein-restricted diet lowered blood urea nitrogen and phosphorus and raised calcium against a low-protein diet alone [1]. In 17 RCTs, they lowered serum phosphate and parathyroid hormone [2].
Population: Adults with CKD not on dialysis [1][2].
Sources: [1], [2]
Caveats: These are blood markers, not fractures or heart events.

Black seed oilModerateHow often is not known

Liver and kidney markers in trials

Liver and kidney markers did not worsen in most trials. Creatinine rose slightly in 3 trials, and doses above 2,000 mg a day were linked to a BUN change.

Creatinine rose by 0.070 in 3 trials with covariate-adjusted analyses, and doses above 2,000 mg a day were linked to a BUN change in 2 trials.

The numbers and sources

How common: How often is not known: the sources give no rate.
Effect: In 19 trials (1,295 participants), Nigella sativa lowered ALP by 10.8 U/L and BUN by 1.016. Creatinine rose by 0.070 in 3 trials with covariate-adjusted analyses, and doses above 2,000 mg a day were linked to a BUN change in 2 trials [11]. In 70 healthy adults given 200 mg a day of a thymoquinone-rich oil for 90 days, liver and kidney tests did not change and no serious adverse effects were reported [12].
Population: Adults in RCTs [11]. 70 healthy adults [12].
Sources: [11], [12]
Caveats: The safety trial's corresponding author is at Akay Natural Ingredients, the maker of the tested oil [12]. Heterogeneity for ALP and BUN was high [11]. In 16 RCTs in type 2 diabetes, AST, ALT and creatinine did not change significantly [7].

Shown to raise it3

High-protein dietStrongHow often is not known

Kidney function in people without kidney disease

Shown to raise kidney filtration rate, urea and urine calcium in people without kidney disease, with no consistent sign of kidney injury in mostly short trials.

The numbers and sources

How common: How often is not known: the sources give no rate.
Effect: Across 30 studies (2,160 people), GFR rose 7.18 mL/min/1.73 m² more on high-protein diets (95% CI 4.45 to 9.91), with higher serum urea (1.75 mmol/L), urinary calcium (25.43 mg a day) and serum uric acid [11]. In 22 RCTs, eGFR rose (SMD 0.39, 95% CI 0.09 to 0.69, I² 74.9%) and serum creatinine did not change (SMD 0.08, 95% CI -0.09 to 0.25) [5]. In 28 trials (1,358 people), GFR after the diet was slightly higher (SMD 0.19, 95% CI 0.07 to 0.31), and the change in GFR did not differ (SMD 0.11, 95% CI -0.05 to 0.27) [9].
Population: Adults without chronic kidney disease in randomized trials [5][9][11].
Sources: [5], [9], [11]
Caveats: Most trials were short and used creatinine-based estimates, so long-term effects are uncertain [5]. Cohorts disagree: in 9,226 Korean adults, the highest protein quarter was linked to 1.32 times the risk of rapid eGFR decline (95% CI 1.02 to 1.73) [12], while a meta-analysis of 6 cohorts (148,051 people) linked higher protein to lower chronic kidney disease incidence (RR 0.82, 95% CI 0.71 to 0.94) [13].

ACE inhibitorsModerateCommon

Kidney function changes

Shown to raise creatinine in some people, with kidney dysfunction in 2.4% on lisinopril against 1.1% of controls after a heart attack.

The numbers and sources

How common: Common: between 1 in 100 and 1 in 10 people.
Effect: Minor, reversible rises in blood urea nitrogen and creatinine occurred in about 2% of people with hypertension on lisinopril alone and in 11.6% of people with heart failure also taking a diuretic. In GISSI-3, kidney dysfunction, defined as creatinine above 3 mg/dL or doubled, occurred in 2.4% on lisinopril and 1.1% of those not given it. In ATLAS, raised creatinine was reported in 10% on the high dose and 7% on the low dose. The label warns that acute kidney failure can occur, particularly with renal artery stenosis, chronic kidney disease, severe heart failure or volume depletion [3].
Population: People in lisinopril clinical trials, including 19,394 in GISSI-3 and 3,164 in ATLAS [3].
Sources: [3]
Caveats: Rises were more frequent with diuretics and often resolved when the diuretic dose was lowered [3]. For the long-term effect on kidney disease, see the kidney outcome block above.

Loop diureticsLimitedCommon

Kidney function

Linked to small rises in creatinine and urea, with raised creatinine in 7.4% of bumetanide patients tested. High furosemide doses caused transient worsening of kidney function in one trial.

The numbers and sources

How common: Common: between 1 in 100 and 1 in 10 people.
Effect: Torsemide 10 mg a day for 6 weeks raised blood urea nitrogen a mean 1.8 mg/dL and creatinine 0.05 mg/dL, reversing after stopping [1]. Raised creatinine was reported in 7.4% and raised urea (azotemia) in 10.6% of patients tested on bumetanide [15]. In acute heart failure, the high-dose furosemide strategy brought transient worsening of kidney function [2].
Population: Hypertensive patients [1]; bumetanide-treated patients [15]; 308 patients [2].
Sources: [1], [15], [2]
Caveats: Creatinine changes during fluid removal can reflect less blood volume rather than kidney damage. No placebo group for the bumetanide figures.

Tested, and not shown to change it1

CreatineStrongHow often is not known

Raised serum creatinine and kidney markers

Shown to raise blood creatinine, which lowers kidney estimates based on it. Kidney function measured another way did not change.

GFR measured with Cr-EDTA, which does not rely on creatinine, did not differ (5.89 mL/min, 95% CI -0.30 to 12.08), nor did urea, albuminuria or proteinuria.

The numbers and sources

How common: How often is not known: the sources give no rate.
Effect: In 26 RCTs (1,036 participants), serum creatinine rose 0.14 mg/dL (95% CI 0.05 to 0.22, I² 93.9%) and creatinine-based GFR estimates fell 10.75 mL/min (95% CI -17.48 to -4.02). GFR measured with Cr-EDTA, which does not rely on creatinine, did not differ (5.89 mL/min, 95% CI -0.30 to 12.08), nor did urea, albuminuria or proteinuria [18]. In 20 randomized trials, serum creatinine rose 0.13 mg/dL (95% CI 0.07 to 0.18), and urea and eGFR did not differ (eGFR -5.20 mL/min/1.73 m², 95% CI -15.00 to 4.60) [19]. In 12 studies (440 participants), serum creatinine rose by a mean of 0.07 (95% CI 0.01 to 0.12), significant at 1 week or less and after 12 weeks but not at 1 to 12 weeks. Glomerular filtration rate did not differ from control [20].
Population: Healthy people and people with chronic kidney disease, including a hemodialysis subgroup [18]; 19 RCTs and one crossover trial [19]; mostly healthy adults, 177 on creatine and 263 controls in the creatinine analysis [20].
Sources: [18], [19], [20]
Caveats: Creatinine is a breakdown product of creatine, so a higher reading and a lower creatinine-based GFR estimate can occur without a change in filtration. The authors of [18] attribute the changes to altered creatinine metabolism, since Cr-EDTA GFR did not change. The 2025 abstract reports its creatinine difference in µmol/L, a unit at which 0.07 would be negligible, and the unit may be a reporting error [20]. The authors of [19] call for trials longer than 1 year.

Type in your own result

How it compares with the reference method

Mixed results1

Limited

Variation from one test to the next

Limited evidence: one study put within-person variation of urea at about 11%, roughly twice that of creatinine in the same people.

The numbers and sources

Compared with: Repeat measurement in the same person
Effect: A study of 730 apparently healthy adults in Bangladesh reported within-person variation of 10.72% for urea against 4.64% for creatinine, variation between people of 31.47%, and a reference change value of 30.58%, the size of change between two results that exceeds expected variation [13]. A European study of 91 healthy adults sampled weekly for 10 weeks reported that urea was one of only two analytes whose within-person variation was not lower than older published estimates, and that it differed between men and women [14].
Population: 730 adults aged 18 to 65 [13]. 91 healthy adults aged 21 to 69 at 6 European laboratories [14].
Sources: [13], [14]
Caveats: The first abstract describes a cross-sectional design and does not say how many samples each person gave [13]. The European abstract gives no figures for urea [14]. Neither states a threshold for agreement.

Did not match the reference method1

Limited

BUN against creatinine-based kidney measures

Limited evidence: in heart and kidney disease cohorts, BUN predicted death or kidney failure beyond what creatinine-based measures explained.

The numbers and sources

Compared with: Serum creatinine, estimated creatinine clearance and eGFR
Effect: In patients admitted for decompensated heart failure, BUN predicted death while creatinine and estimated creatinine clearance did not after adjustment for other factors [6]. In older Medicare patients, one creatinine-based eGFR equation failed to identify increased risk in 50% of heart attack patients and 60% of heart failure patients, and BUN carried the greatest relative importance for predicting death among the kidney measures compared [7]. In people with chronic kidney disease, BUN was associated with kidney outcomes independent of eGFR [9]. In US veterans, eGFR was not associated with new diabetes when BUN was 25 mg/dL or less, while BUN above 25 mg/dL was, at every level of eGFR [8].
Population: 541 patients with heart failure [6]. 101,089 Medicare patients [7]. 459 patients with chronic kidney disease [9]. 1,337,452 US veterans [8].
Sources: [6], [7], [9], [8]
Caveats: None of these studies compared BUN with directly measured GFR, so they show that BUN and creatinine carry different information, not which is closer to true filtration. The ratio of BUN to creatinine is a separate quantity. In one US survey analysis its association with death from any cause was U-shaped, lowest at a ratio of 11.43 to 14.64 [12]. The ratio is not placed as a range on this page.

What studies tie it to

Linked in research5

Limited

Death from any cause in the general population

One US cohort found 48% more deaths from any cause in the highest quarter of BUN than in the lowest over about 12 years.

The numbers and sources

Effect: In 17,719 US adults followed for a mean of 11.65 years, people in the highest quarter of BUN (5.71 mmol/L or above, about 16 mg/dL) had more deaths from any cause than those in the lowest quarter (below 3.57 mmol/L, about 10 mg/dL), with an adjusted hazard ratio of 1.48 (95% CI 1.28 to 1.72). The hazard ratio for cardiovascular death was 1.48 (1.08 to 2.02). In subgroups the trend held most clearly in women [5].
Population: 17,719 adults in the US National Health and Nutrition Examination Survey, 1999 to 2006, with 3,628 deaths, 859 from cardiovascular disease [5].
Sources: [5]
Caveats: Observational, one cohort, one BUN measurement per person. The models adjusted for demographics, lifestyle, hypertension and diabetes. The abstract does not list kidney function among the adjustments, and the test is used as a marker of kidney disease [2][5].

Moderate

Death in heart failure and after a heart attack

Linked to more deaths in people hospitalized with heart failure, about 2.3 times the risk in the top quarter of BUN in one cohort.

The numbers and sources

Effect: In 541 patients admitted for decompensated heart failure, the risk of death rose with each quartile of BUN, with an adjusted relative risk of 2.3 (95% CI 1.3 to 4.1) in the top against the bottom quartile [6]. In a US registry of heart failure admissions, a BUN of 43 mg/dL or above on admission was the single strongest predictor of death in hospital among 39 variables examined [4]. In older Medicare patients, adjusted 1-year mortality rose 3% per 5 mg/dL of BUN after a heart attack and 7% per 5 mg/dL in heart failure, with risk climbing steadily above 17 mg/dL and 16 mg/dL [7].
Population: 541 patients, mean age 63, followed for a mean of 343 days [6]. 33,046 hospitalizations in the derivation cohort and 32,229 in the validation cohort, mean age 72.5 [4]. 44,437 patients hospitalized for heart attack and 56,652 for heart failure, aged 65 or older [7].
Sources: [6], [4], [7]
Caveats: Observational, in people already in hospital with heart disease. The authors of the first study suggest that a raised BUN in this setting reflects reduced blood flow to the kidneys from the heart failure itself [6]. These cut points were derived to predict death in hospital patients and are not reference ranges.

Limited

Type 2 diabetes

One cohort of 1.3 million US veterans found 23% more new diabetes with BUN above 25 mg/dL than at 25 or below.

The numbers and sources

Effect: In 1,337,452 US veterans without diabetes followed for a median of 4.93 years, a BUN over 25 mg/dL went with more new diabetes than a BUN of 25 mg/dL or less (hazard ratio 1.23, 95% CI 1.21 to 1.25), with eGFR in the model. The association held at every level of eGFR, including 60 mL/min/1.73 m2 or more (hazard ratio 1.27, 95% CI 1.24 to 1.31). Each 10 mg/dL higher BUN went with a hazard ratio of 1.15 (1.14 to 1.16) [8].
Population: 1,337,452 US veterans, with 172,913 new cases of diabetes [8].
Sources: [8]
Caveats: Observational, one cohort of US veterans. The cut point of 25 mg/dL was the study's analysis category, not a reference range [8].

Limited

Kidney failure in people with chronic kidney disease

One study of 459 people with stage 3 to 5 kidney disease found more kidney failure or death in the top quarter of BUN, independent of eGFR.

The numbers and sources

Effect: In 459 Japanese patients with chronic kidney disease stages 3 to 5 followed for a median of 25.8 months, hazard ratios for end-stage renal disease or death in the second, third and fourth quartiles of BUN against the first were 1.36 (95% CI 0.72 to 2.58), 1.87 (0.95 to 3.66) and 2.66 (1.23 to 5.76), after adjustment that included eGFR. Higher BUN quartiles also carried more risk of end-stage renal disease alone [9].
Population: 459 patients with chronic kidney disease stages 3 to 5, of whom 210 reached the endpoint [9].
Sources: [9]
Caveats: Observational, one cohort in one country. Only the top quartile differed significantly from the bottom [9].

Moderate

Death in people hospitalized with pneumonia or pancreatitis

Linked to more deaths in hospital patients: a BUN of 20 mg/dL or above went with 4.6 times the odds of death in acute pancreatitis.

The numbers and sources

Effect: In 1,043 cases of acute pancreatitis from 3 cohorts, a BUN of 20 mg/dL or higher at admission went with an odds ratio for death of 4.6 (95% CI 2.5 to 8.3), and any rise in BUN at 24 hours with an odds ratio of 4.3 (2.3 to 7.9) [10]. In 1,068 adults hospitalized with community-acquired pneumonia, urea above 7 mmol/L (a BUN above about 20 mg/dL) became one of the five items of the CURB-65 severity score, in which 30-day mortality ran from 0.7% at a score of 0 to 57% at a score of 5 [11].
Population: 1,043 cases of acute pancreatitis in US and Dutch cohorts [10]. 1,068 patients with pneumonia in the UK, New Zealand and the Netherlands, mean age 64 [11].
Sources: [10], [11]
Caveats: Observational, in people acutely ill in hospital. These cut points are not reference ranges for people who are well. The pneumonia study reports urea in mmol/L, and the mg/dL figure here is converted with the factor on this page.

Sources

US Food and Drug Administration. 21 CFR 862.1770, Urea nitrogen test system. Electronic Code of Federal Regulations. https://www.ecfr.gov/current/title-21/section-862.1770
Centers for Disease Control and Prevention, National Center for Health Statistics. National Health and Nutrition Examination Survey 2017-2018 Laboratory Procedure Manual: Blood Urea Nitrogen (BUN), serum, Roche Cobas 6000. Performed by the University of Minnesota Advanced Research and Diagnostic Laboratory. https://wwwn.cdc.gov/nchs/data/nhanes/public/2017/labmethods/BIOPRO-J-MET-BUN-508.pdf
American Board of Internal Medicine. ABIM Laboratory Test Reference Ranges, January 2026. https://www.abim.org/Media/bfijryql/laboratory-reference-ranges.pdf
Fonarow GC, Adams KF, Abraham WT, et al. Risk Stratification for In-Hospital Mortality in Acutely Decompensated Heart Failure: Classification and Regression Tree Analysis. JAMA. 2005. https://doi.org/10.1001/jama.293.5.572
Hong C, Zhu H, Zhou X, et al. Association of Blood Urea Nitrogen with Cardiovascular Diseases and All-Cause Mortality in USA Adults: Results from NHANES 1999-2006. Nutrients. 2023. https://doi.org/10.3390/nu15020461
Aronson D, Mittleman MA, Burger AJ. Elevated blood urea nitrogen level as a predictor of mortality in patients admitted for decompensated heart failure. The American Journal of Medicine. 2004. https://doi.org/10.1016/j.amjmed.2003.11.014
Smith GL, Shlipak MG, Havranek EP, et al. Serum Urea Nitrogen, Creatinine, and Estimators of Renal Function: Mortality in Older Patients With Cardiovascular Disease. Archives of Internal Medicine. 2006. https://doi.org/10.1001/archinte.166.10.1134
Xie Y, Bowe B, Li T, et al. Higher blood urea nitrogen is associated with increased risk of incident diabetes mellitus. Kidney International. 2018. https://doi.org/10.1016/j.kint.2017.08.033
Seki M, Nakayama M, Sakoh T, et al. Blood urea nitrogen is independently associated with renal outcomes in Japanese patients with stage 3-5 chronic kidney disease: a prospective observational study. BMC Nephrology. 2019. https://doi.org/10.1186/s12882-019-1306-1
Wu BU, Bakker OJ, Papachristou GI, et al. Blood Urea Nitrogen in the Early Assessment of Acute Pancreatitis: An International Validation Study. Archives of Internal Medicine. 2011. https://doi.org/10.1001/archinternmed.2011.126
Lim WS, van der Eerden MM, Laing R, et al. Defining community acquired pneumonia severity on presentation to hospital: an international derivation and validation study. Thorax. 2003. https://doi.org/10.1136/thorax.58.5.377
Shen S, Yan X, Xu B. The blood urea nitrogen/creatinine (BUN/cre) ratio was U-shaped associated with all-cause mortality in general population. Renal Failure. 2022. https://doi.org/10.1080/0886022X.2022.2030359
Ali MA, Hossain MS, Juliana FM, Reza MS. Evaluation of Biological Variation of Different Clinical Laboratory Analytes in the Blood of Healthy Subjects. Cureus. 2023. https://doi.org/10.7759/cureus.36242
Aarsand AK, Díaz-Garzón J, Fernandez-Calle P, et al. The EuBIVAS: Within- and Between-Subject Biological Variation Data for Electrolytes, Lipids, Urea, Uric Acid, Total Protein, Total Bilirubin, Direct Bilirubin, and Glucose. Clinical Chemistry. 2018. https://doi.org/10.1373/clinchem.2018.288415

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