Kidney function involves removing metabolic waste from the bloodstream, regulating water balance in the body, and maintaining the acidity/alkalinity of body fluids. Both blood and urine tests can determine if kidneys are functioning properly. Blood tests that assess kidney function are often ordered along with liver function tests to monitor patients who are being treated with certain arthritis medications.
Kidney toxicity is a potential side effect of some arthritis medications. Kidney problems that develop from the use of arthritis medications are not common, but they can occur. Doctors may routinely order tests to monitor the effect of certain drugs on the kidneys, screening for kidney problems or abnormalities with kidney function.
Blood Tests for Kidney Function
BUN (Blood Urea Nitrogen) is a blood test which assesses kidney function. Urea is a by-product of protein metabolism and is formed in the liver. Urea is filtered from the blood by the kidneys and excreted in the urine.
Arthritis or gout drugs that can increase BUN levels include:
Gastrointestinal bleeding which can be a serious side effect of NSAIDs can cause elevation of BUN.
BUN normal range is 7-20 mg/dl
Serum creatinine is a blood test which is also used to evaluate kidney function. Creatinine is a by-product of creatine which is involved with muscle energy metabolism. A relatively small amount of creatine in the body is converted to creatinine daily. Creatinine, the metabolic waste product from the conversion, is filtered from the blood by the kidneys and excreted into urine.
Blood creatinine is normally kept quite stable by the kidneys. Creatinine levels in the blood become elevated if kidney function is impaired. Although age and gender affect normal values of creatinine, the stability of a person's blood creatinine level make it a preferred test to BUN for evaluating kidney problems.
Serum creatinine normal range is 0.8-1.2 mg/dl (male) and 0.6-0.9 mg/dl (females).
Urine Tests for Kidney Function
Urinalysis is the most simple of urine tests which are used to screen for kidney problems. A simple dipstick can detect the presence of glucose, protein, ketones, or bilirubin in the urine as well as the acidity or alkalinity of the urine, indicators of potential kidney problems. The appearance of the urine is always noted with a urinalysis. Cellular abnormalities can be detected microscopically.
Urea clearance is a test which uses a blood specimen to determine the level of urea present in the blood and two urine samples, the second being collected one hour after the first urine sample. This test detects how much urea is filtered by the kidneys into the urine.
Urea clearance normal range is 64-99 ml/min
Creatinine clearance, also measured as ml/min, compares the level of creatinine in urine with the creatinine level in the blood, usually based on measurements of a 24-hour urine sample and a blood sample drawn at the end of the 24-hour period, according to ADAM.
As previously stated, creatinine is found in stable plasma concentrations. When creatinine is filtered, it is not reabsorbed, and is minimally secreted by the kidneys. The creatinine clearance is used to estimate the glomerular filtration rate, a standard assessment of kidney function.
Creatinine clearance normal range/24 hour urine for adults <40 years old are 90-139 ml/min (males), 80-125 ml/min (females). Over 40 years old normal decreases 6.5 ml/min for each decade of life.
Urine osmolality is a measure of urine concentration based on the number of particles disolved into the urine. Measured in milliosmols/kg.
Urine osmolality normal ranges are 50 to 1400 mOsm/kg (random) or greater than 850 mOsm/kg for 12 to 14 hour fluid restriction.
Urine protein test is a 24 hour collection of urine.
Normal = the sample should contain less than or equal to 150 mg of protein.
Abnormal Kidney Tests Point to Kidney Problems
Any tests found to be out of normal range are indicators of potential kidney problems. You, along with your doctor must decide if one of your arthritis medications is causing the problem. Stopping or switching a medication may possibly be the next best course, allowing kidney problems to resolve.
All normal ranges are dependent on the laboratory and method used for testing
BUN, Creatinine, Osmolality, Urinalysis, from ADAM.
Kidney Function Tests, HealthAtoZ.
Creatinine, Blood Urea Nitrogen, Osmolality, from MedicineNet.
Clinical Diagnosis By Laboratory Methods, Todd-Sanford, 15th edition, 1974.