The BUN\/creatinine ratio compares two waste products that appear side by side on a standard metabolic panel: blood urea nitrogen (BUN) and creatinine. Both are cleared by the kidneys, but they come from different places and respond to different things, so looking at them together tells your doctor more than either number alone. A high ratio often points to something outside the kidneys, such as dehydration, a high-protein diet or bleeding in the upper digestive tract. A low ratio can reflect liver disease, a low protein intake or a high muscle mass. The ratio is a clue, not a diagnosis.<\/p>
In this article you’ll learn how the ratio is calculated, what counts as a typical value, the common reasons for a high or low ratio, how it relates to kidney function, and when a result needs medical attention.<\/p>
BUN measures urea nitrogen, a waste product that forms when your body breaks down protein; the kidneys remove it in urine, according to MedlinePlus<\/a>. Creatinine is a waste product released as muscles are used, and the amount you make depends mostly on your muscle mass, age and activity level, as MedlinePlus<\/a> explains. Both are filtered by the kidneys, but BUN is also strongly affected by how much protein you eat, how well hydrated you are and whether there is blood in your gut, while creatinine stays steadier.<\/p> That difference is what makes the ratio useful. If both values rise together, the kidneys themselves may be filtering less. If BUN rises much more than creatinine, the cause often lies elsewhere. Both tests are part of a routine metabolic panel, and MedlinePlus lists the BUN to creatinine ratio as another name for the BUN test. To review each value in detail, read our BUN blood test guide<\/a>, check our gu\u00eda del an\u00e1lisis de creatinina en sangre<\/a> and open our comprehensive metabolic panel guide<\/a>.<\/p> The calculation is simple: divide the BUN by the creatinine. With a BUN of 18 mg\/dL and a creatinine of 0.9 mg\/dL, the ratio is 18 \u00f7 0.9 = 20. Many US reports calculate it automatically and print it on the line below the two results.<\/p> Units change the number completely. In the United States, both values are usually reported in milligrams per deciliter (mg\/dL). Many other countries measure urea in millimoles per liter and creatinine in micromoles per liter, which produces much larger numbers for the same person. A 2025 review of hospital studies noted that a BUN-to-creatinine ratio of 20 in US units corresponds to a urea-to-creatinine ratio of about 80 when both are converted to millimoles per liter (Paulus et al., 2025). Always check which units your report uses before comparing it with ranges you read online.<\/p> There is no single standard range for the BUN\/creatinine ratio. Some laboratories print a reference range next to it, and that is the range to use. In research using US units, a value around 20 is commonly used as the dividing line between a typical and a raised ratio. For context, Mayo Clinic<\/a> describes a BUN of about 7 to 20 mg\/dL as generally healthy, and Cleveland Clinic<\/a> gives adult ranges of 6 to 21 mg\/dL for women and 8 to 24 mg\/dL for men; creatinine ranges depend heavily on muscle mass and sex.<\/p> Because the ratio divides one number by another, it can look abnormal when both values are normal. A slightly high BUN combined with a low creatinine, common in small or older adults with less muscle, can produce a ratio above 20 without any illness. The pattern of the two numbers matters more than the ratio alone, as the table shows.<\/p>How the ratio is calculated<\/h3>
Normal range for the BUN\/creatinine ratio<\/h2>