Kidney Function Test

Urinalysis

Urinalysis screens for the presence of protein and blood in the urine. There are many possible reasons for protein in your urine, not all of which are related to the disease. Infection increases urine protein, but so does a heavy physical workout. Your doctor may want to repeat this test after a few weeks to see if the results are similar.

Your doctor may also ask you to provide a 24-hour urine collection sample. This can help doctors see how fast a waste product called creatinine is clearing from your body. Creatinine is a breakdown product of muscle tissue.

Normal Urine Characteristics 
                  Characteristic                                              Normal values
                           Color                                       Pale yellow to deep amber
                           Odor                                                     Odorless
                         Volume                                       750–2000 mL/24 hour
                             pH                                                     4.5–8.0
                 Specific gravity                                               1.003–1.032
                     Osmolarity                                        40–1350 mOsmol/kg
                   Urobilinogen                                          0.2–1.0 mg/100 mL
                White blood cells                0–2 HPF (per high-power field of the microscope)
              Leukocyte esterase                                                      None
                         Protein                                               None or trace
                        Bilirubin                                          <0.3 mg/100 mL
                         Ketones                                                    None
                        Nitrites                                                    None
                         Blood                                                    None
                       Glucose                                                    None

Serum creatinine test

This blood test examines whether creatinine is building up in your blood. The kidneys usually completely filter creatinine from the blood. A high level of creatinine suggests a kidney problem.

According to the National Kidney Foundation (NKF), a creatinine level higher than 1.2 milligrams/deciliter (mg/dL) for women and 1.4 mg/dL for men is a sign of a kidney problem.

Blood urea nitrogen (BUN)

The blood urea nitrogen (BUN) test also checks for waste products in your blood. BUN tests measure the amount of nitrogen in the blood. Urea nitrogen is a breakdown product of protein.

However, not all elevated BUN tests are due to kidney damage. Common medications, including large doses of aspirin and some types of antibiotics, can also increase your BUN. It’s important to tell your doctor about any medications or supplements that you take regularly. You may need to stop certain drugs for a few days before the test.

A normal BUN level is between 7 and 20 mg/dL. A higher value could suggest several different health problems.

Estimated GFR

This test estimates how well your kidneys are filtering waste. The test determines the rate by looking at factors, such as:

  • test results, specifically creatinine levels
  • age
  • gender
  • race
  • height
  • weight

Any result lower than 60 milliliters/minute/1.73m2 may be a warning sign of kidney disease.

                                                                         Urine Volumes
        Volume
Condition
   Volume                                                              Causes
   Normal    1–2 L/day
   Polyuria     >2.5 L/day Diabetes mellitus; diabetes insipidus; excess caffeine or alcohol; kidney disease; certain drugs, such as diuretics; sickle cell anemia; excessive water intake
   Oliguria   300–500 mL/day Dehydration; blood loss; diarrhea; cardiogenic shock; kidney disease; enlarged prostate
     Anuria <50 mL/day Kidney failure; obstruction, such as kidney stone or tumor; enlarged prostate

How the tests are Performed

Kidney function tests usually require a 24-hour urine sample and a blood test.

24-hour urine sample

A 24-hour urine sample is a creatinine clearance test. It gives your doctor an idea of how much creatinine your body expels over a single day.

On the day that you start the test, urinate into the toilet as you normally would when you wake up.

For the rest of the day and night, urinate into a special container provided by your doctor. Keep the container capped and refrigerated during the collection process. Make sure to label the container clearly and to tell other family members why it’s in the refrigerator.

On the morning of the second day, urinate into the container when you get up. This completes the 24-hour collection process.

Follow your doctor’s instructions about where to drop the sample off. You may need to return it either to your doctor’s office or a laboratory.

Blood samples

BUN and serum creatinine tests require blood samples taken in a lab or doctor’s office.

The technician drawing the blood first ties an elastic band around your upper arm. This makes the veins stand out. The technician then cleans the area over the vein. They slip a hollow needle through your skin and into the vein. The blood will flow back into a test tube that will be sent for analysis.

You may feel a sharp pinch or prick when the needle enters your arm. The technician will place gauze and a bandage over the puncture site after the test. The area around the puncture may develop a bruise over the next few days. However, you shouldn’t feel severe or long-term pain.

 

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