When going to the doctor, you may go through a battery of exams to assess your overall health. Kidney tests, especially for individuals with diabetes or hypertension, are important for assessing the health of your kidneys. The National Kidney Foundation states that blood pressure, glomerular filtration rate and urinary analysis can be used to test how well the kidneys are working. Creatinine, which is produced by the muscles, can be used for the measurement of glomerular filtration rate and urinary analysis.
Kidney Function
The kidneys are two bean-shaped organs located in the middle of your back whose major function is to filter your blood, removing waste from the body. If your kidneys are not functioning properly, waste can build in the body, leading to death. One of the best ways to assess kidney function is by measuring your glomerular filtration rate or GFR, which measures the rate your body is able to filter out waste products. GFR can be assessed directly or indirectly by measuring the levels of markers in the blood or urine. Direct measures are costly and more difficult to perform, making them less practical in most clinical or research settings. Creatinine is a widely used indirect measure of GFR because of its low cost and ease to assess.
Creatinine
Creatinine is the product of muscle breakdown and is usually produced at a constant rate. Creatinine is freely filtered by the kidney, which allows it to be used to assess kidney function. However, since creatinine is produced by the muscles, creatinine values can be influence greatly by muscle mass. Muscle mass can greatly differ by age, gender, race and meat consumption, all of which can influence the interpretation of creatinine values. Formulas have been created to account for differences in muscle to provide better estimation of kidney function.
GFR Values
GFR values from serum creatinine can be used to classify stage of kidney disease or function. However, creatinine alone should not be used to assess kidney function. Other markers of kidney damage, such as the presence of blood or protein in the urine, provide valuable information. The National Kidney Foundation provides guidelines for how to determine kidney function using creatinine estimated GFR and markers of kidney damage. Normal functioning kidneys have a GFR between 90 and 130 and no other markers of kidney damage. Stage 1 kidney disease is defined as having a GFR between 90 and 130, with the presence of kidney damage. Stage 2 kidney disease is defined as having a GFR between 60 and 90. A GFR between 30 and 60 is defined as stage 3 kidney disease. Renal insufficiency or stage 4 kidney disease is means having a GFR between 15 and 30. A GFR below 15 is known as stage 5 kidney disease or kidney failure.
Urinary Analysis
Kidney function can also be estimated as a ratio of urinary albumin to urinary creatinine. Albumin is a protein that the kidneys should keep from being excreted in urine. The presence of urinary albumin indicates kidney dysfunction. In properly functioning kidneys, the albumin to creatinine ratio should be below 30 mg/g. An albumin to creatinine ratio above 30 mg/g may be an early sign of kidney damage.
