Because a person can have kidney disease without any symptoms, a doctor may first detect the condition through routine blood and urine tests. The National Kidney Foundation recommends three simple tests to screen for kidney disease: a blood pressure measurement, a spot check for protein or albumin in the urine, and a calculation of glomerular filtration rate (GFR) based on a serum creatinine measurement. Measuring urea nitrogen in the blood provides additional information.
Blood Pressure Measurement
High blood pressure can lead to kidney disease. It can also be a sign that the kidneys are already impaired. The only way to know whether a person’s blood pressure is high is to have a health professional measure it with a blood pressure cuff. The result is expressed as two numbers. The top number, which is called the systolic pressure, represents the pressure in the blood vessels when the heart is beating. The bottom number, which is called the diastolic pressure, shows the pressure when the heart is resting between beats. A person’s blood pressure is considered normal if it stays below 120/80, stated as “120 over 80.” The NHLBI recommends that people with kidney disease use whatever therapy is necessary, including lifestyle changes and medicines, to keep their blood pressure below 130/80.
Microalbuminuria and Proteinuria
Healthy kidneys take wastes out of the blood but leave protein. Impaired kidneys may fail to separate a blood protein called albumin from the wastes. At first, only small amounts of albumin may leak into the urine, a condition known as microalbuminuria, a sign of deteriorating kidney function. As kidney function worsens, the amount of albumin and other proteins in the urine increases, and the condition is called proteinuria. A doctor may test for protein using a dipstick in a small sample of a person’s urine taken in the doctor’s office. The color of the dipstick indicates the presence or absence of proteinuria.
A more sensitive test for protein or albumin in the urine involves laboratory measurement and calculation of the protein-to-creatinine or albumin-to-creatinine ratio. Creatinine is a waste product in the blood created by the normal breakdown of muscle cells during activity. Healthy kidneys take creatinine out of the blood and put it into the urine to leave the body. When the kidneys are not working well, creatinine builds up in the blood.
The albumin-to-creatinine measurement should be used to detect kidney disease in people at high risk, especially those with diabetes or high blood pressure. If a person’s first laboratory test shows high levels of protein, another test should be done 1 to 2 weeks later. If the second test also shows high levels of protein, the person has persistent proteinuria and should have additional tests to evaluate kidney function.
Glomerular Filtration Rate (GFR) Based on Creatinine Measurement
GFR is a calculation of how efficiently the kidneys are filtering wastes from the blood. A traditional GFR calculation requires an injection into the bloodstream of a substance that is later measured in a 24-hour urine collection. Recently, scientists found they could calculate GFR without an injection or urine collection. The new calculation—the eGFR—requires only a measurement of the creatinine in a blood sample.
In a laboratory, a person’s blood is tested to see how many milligrams of creatinine are in one deciliter of blood (mg/dL). Creatinine levels in the blood can vary, and each laboratory has its own normal range, usually 0.6 to 1.2 mg/dL. A person whose creatinine level is only slightly above this range will probably not feel sick, but the elevation is a sig...