Kidney Function Tests Health Article

Preparation

Patients will be given specific instructions for collection of urine samples, depending on the test to be performed. During routine urinalysis, the patient will be given a sealed cup to urinate into. Nurses stress that the patient obtain a "clean catch" by initiating urination and placing the sample cup in the urine stream after a few seconds. This prevents the collection of the initial urine which may contain bacteria that are present in the lower urethra or on the skin. Some timed urine tests require an extended collection period of up to 24 hours, during which time the patient collects all urine voided and transfers it to a specimen container. Refrigeration and/or preservatives are typically required to maintain the integrity of such urine specimens. Certain dietary and/or medication restrictions may be imposed for some of the blood and urine tests. The patient may also be instructed to avoid exercise for a period of time before a test to prevent changes in creatinine.

Aftercare

If medication was discontinued prior to a kidney function test, it may be resumed once the test is completed.

Complications

Complications for these tests are minimal, but may include slight bleeding from a venipuncture site, hematoma (accumulation of blood under a puncture site), or fainting or feeling light-headed after venipuncture. In addition, suspension of medication or dietary changes imposed in preparation for some blood or urine tests may trigger side-effects in some individuals.

Results

Normal values for many tests are determined by the patient's age and sex. Reference values can also vary by laboratory, but are generally within the ranges that follow.

Urine tests

• Creatinine clearance. For a 24-hour urine collection, normal results are 90-139 mL/min for adult males less than 40 years old, and 80-125 mL/min for adult females less than 40 years old. For people over 40, values decrease by 6.5 mL/min for each decade of life.
• Urine osmolality. With restricted fluid intake (concentration testing), osmolality should be greater than 800 mOsm/kg of water. With increased fluid intake (dilution testing), osmolality should be less than 100 mOSm/kg in at least one of the specimens collected. A 24-hour urine osmolality should average 300-900 mOsm/Kg. A random urine osmolality should average 500-800 mOsm/Kg.
• Urine protein. A 24-hour urine collection should contain no more than 150 mg of protein.
• Urine sodium. A 24-hour urine sodium should be within 75-200 mmol/day.

Blood tests

• Blood urea nitrogen (BUN) should average 8-20 mg/dL.
• Creatinine should be 0.8-1.2 mg/dL for males, and 0.6-0.9 mg/dL for females.
• Uric acid levels for males should be 3.5-7.2 mg/dL and for females 2.6-6.0 mg/dL.

Low clearance values for creatinine indicate diminished ability of the kidneys to filter waste products from the blood and excrete them in the urine. As clearance levels decrease, blood levels of creatinine, urea, and uric acid increase. Since it can be affected by other factors, an elevated BUN, by itself, is suggestive, but not diagnostic, for kidney dysfunction. An abnormally elevated plasma creatinine is a more specific indicator of kidney disease than is BUN.

Inability of the kidneys to concentrate the urine in response to restricted fluid intake, or to dilute the urine in response to increased fluid intake during osmolality testing indicates decreased tubular function. Because the kidneys normally excrete almost no protein in the urine, its persistent presence, in amounts that exceed the normal 24-hour urine value, usually indicates glomerular or tubular injury. These can be distinguished by urine protein electrophoresis. This procedure separates proteins in an electric field based upon their charge. Albuminuria is characteristic of glomerular disease, while urinary excretion of alpha-1 and beta-2 microglobulins is characteristic of tubular damage. Proteinuria of tubular origin is caused by drugs, heavy metals, or viral infection of the kidneys. Urine protein electrophoresis also detects monoclonal immunoglobulin light chains (multiple myeloma and related conditions) and immunoglobulin fragments (systemic autoimmune diseases), which are nonrenal causes of proteinuria.

Health care team roles

Kidney function tests are ordered and interpreted by a physician. Blood samples are collected by a nurse or phlebotomist. Nurses should educate the patient on why the tests are being done and how to collect timed urine samples. In addition, patients with kidney disease may be advised to change their diets. A dietitian may be consulted.

Patient education

Some kidney problems are the result of another disease process such as diabetes or high blood pressure. Clinicians should take the time to inform patients about how their disease or its treatment will alter kidney function and the different measures they can take to help prevent these changes.