BUN/creatinine ratio: comparing two kidney markers

Healthy kidneys filter waste from the blood into urine. Two common waste products are urea and creatinine. Lab Tests Online UK explains that urea and creatinine are often the first tests used to check how well the kidneys filter, and that urea is also called BUN. The ratio is made by dividing one by the other, and it is most often quoted in US-style reports where BUN and creatinine are both in mg/dL.

UK laboratories normally give urea in millimoles per litre and creatinine in micromoles per litre, so the same numerical ratio does not apply. Clinicians here more commonly talk about whether urea is raised out of proportion to creatinine.

Creatinine is made by muscles from creatine at a steady rate and depends on muscle mass; Lab Tests Online UK notes men usually have slightly higher levels than women and levels are lower in children and older people. Urea comes from protein breakdown, so it rises with a high-protein diet, with bleeding in the gut, and with reduced blood flow to the kidneys.

This is why they can diverge. Dehydration slows kidney filtering and also increases the reabsorption of urea, so urea tends to rise more than creatinine. Heavy upper gut bleeding adds a protein load, again lifting urea more. Muscle injury (rhabdomyolysis) pushes creatinine up. Low muscle mass in frailty can hide a fall in kidney function by keeping creatinine low.

A ratio taken from one blood sample is a snapshot. Doctors look at the trend over days and weeks, how the person looks clinically, urine output, blood pressure, medicines and recent illness. For kidney function itself the eGFR derived from creatinine, and the urine albumin-to-creatinine ratio, are the accepted measures: NICE guidance cited by Lab Tests Online UK states that UK laboratories should report an eGFR with every creatinine result.

The eGFR is less reliable when kidney function is changing quickly, as in acute kidney injury, and in people with very high or low muscle mass, so clinicians combine several clues.

You do not need to fast. Eating meat just before a creatinine test may temporarily raise it, and Lab Tests Online UK suggests avoiding meat for at least 12 hours beforehand; creatine supplements should be mentioned. NICE advises avoiding meat or fish for the 12 hours before an eGFR blood test, according to the same source.

If urea is out of proportion, the clinician may ask about fluid intake, vomiting, diarrhoea, black stools and medicines such as anti-inflammatories and water tablets. Rehydration and a repeat test are common. If the gut is bleeding, urgent assessment follows. If both markers stay high, the next steps include a urine albumin-to-creatinine ratio, a kidney ultrasound and referral for assessment.

Seek urgent help for passing very little urine, severe vomiting that prevents drinking, black or bloody stools, chest pain or confusion; call NHS 111, or 999 in an emergency.