Mercy Kidney Score

Understanding Your Kidney Score

By W. A. Wilmer, MD, John D. Mahan, MD

Kidneys

The main function of the kidneys is to remove waste products and extra salts and water from your body. The kidneys have other functions (see below) that are affected in kidney diseases.

Signs of Kidney Disease

Pain and discomfort are usually not symptoms of most kidney diseases. Most people can not tell when their kidneys are sick. By the time symptoms of the build up of waste products occurs, kidney disease may have been present for months or years, and the ability to treat the disease may be limited.

Screening for kidney disease is relatively quick and inexpensive. Blood and urine tests can tell you if kidney disease is present before you feel ill.

The Kidney Score is a major way you can determine your kidney health.

A Quick Anatomy and Physiology Lesson

Your kidneys are approximately 12 centimeters in length and are located near the middle of your back, just below the rib cage.

Your kidneys process about 200 quarts of blood to create about 2 quarts of fluid, waste products, excessive salts, calcium and phosphates (called urine) each day.

The urine flows to your bladder through tubes called ureters. Your bladder stores urine until it passes through the urethra, out of your body.

The waste products are from the normal breakdown of active tissues and from the food you eat. If your kidneys do not remove these wastes, the wastes build up in the blood and damage your body.

The cleaning of wastes from your blood occurs in filtering units called nephrons (that is why a kidney specialist is called a "nephrologist"). In most people, each kidney has 800,000 to one million nephrons. In the nephron, a small ball of capillaries called a glomerulus is where wastes, salts and water are squeezed from the circulation.

The fluid that leaves the circulation is an early form of urine. This fluid enters a specialized plumbing system called a tubule. Since this early form of urine also contains water and salts that your body may still need, the tubules absorb some parts of the urine back into your bloodstream.

The kidneys sense minute-by-minute what chemistries your body needs to fine-tune how much water and salts leave in the urine with the wastes you do not need.

In addition to removing wastes, your kidneys release three important hormones:

erythropoietin (eh-RITH-ro-POY-eh-tin), or EPO, which stimulates the bone marrow to make red blood cells. In kidney diseases, red blood cell production may decline, leading to anemia.
renin (REE-nin), which regulates blood pressure.
calcitriol (kal-suh-TRY-ul), the active form of vitamin D, which helps maintain calcium for bones and for normal chemical balance in the body. In kidney diseases, calcium levels and bone health may suffer due to a lack of activated vitamin D.

What is the Kidney Score?

Your Kidney Score is the measure of your kidneys ability to filter wastes from the bloodstream.

If you have two healthy kidneys, you have 100% of your kidney filtering ability. This is more renal function than you really need. For example, some people are born with only one kidney, and these people are able to lead normal, healthy lives. Many people donate a kidney for transplantation to a family member or friend and survive well using only the remaining kidney.

Some people with low kidney function have a kidney disease that will get worse. You will have serious health problems if you have less than 25% of your renal function. If your renal function drops below 10 to 15%, you may not be able to live a normal life without some form of renal replacement therapy — either dialysis or kidney transplantation.

Most kidney diseases attack the nephrons, causing them to lose their filtering capacity. Most kidney diseases destroy the nephrons slowly and silently over time. Most kidney diseases attack both kidneys simultaneously.

A blood test — called the creatinine — can help determine how well your kidneys are filtering. Creatinine is a waste product generated by skeletal muscle that is easily filtered by the kidneys, along with other waste products. If the kidneys are filtering normally, blood creatinine levels should be low. As the kidney filtering ability decreases, blood creatinine levels increase.

The kidney score represents the ability of the glomeruli to filter the blood, otherwise known as the glomerular filtration rate, or GFR. For an adult the normal GFR should be around 100. A limitation of only using a blood creatinine level to check your kidney health is the fact that creatinine is a byproduct of muscle metabolism, and the amount of muscle varies in people.

The simplest kidney score calculation uses your blood creatinine level, age, gender, and race in a mathematical formula that was developed during the clinical study called the MDRD study. The MDRD equation calculates GFR better than any other equations used in the past to estimate GFR. There are limitations of this measurement that you should know (see below), but currently this is the best test for identifying low kidney function.

Limitations of the Kidney Score Using the MDRD Equation

The equation used to calculate your kidney score was developed from an NIH-sponsored clinical study that followed hundreds of patients with different types of chronic kidney disease. Very accurate research tests were performed that to develop the equation that estimates GFR. Using age, gender, race and serum creatinine levels, this equation is fairly accurate.
This equation is most accurate if your GFR is < 60% normal. At GFRs above 60% of normal, the equation is less accurate, and often another measurement of GFR (e.g. a 24-hour collection of urine to measure creatinine filtration) will be more accurate to determine the true GFR. However, most patients only need to know when their "kidney score" is < 60% — that is the cutoff that requires an active investigation into why your kidney function is not normal.

Also, this MDRD equation is not as accurate if you:

Are obese or thinner than average (a body surface area, or BSA, > 28 or < 20).
If you are younger than 18 years of age
If you are taking certain medications or body-building supplements (e.g. fenofibrate, or creatine supplements).

If the kidney score is slightly less than 100 (say between 60 and 100), your kidney function may be normal! Simple measures such as better hydration (drinking more fluids) before repeating the test may reveal a kidney score closer to 100.

Remember, the kidney score is an estimation of GFR or kidney function and more accurate determinations of kidney function through more involved urine and blood tests can be performed by your physician.

Stages of Chronic Kidney Disease (CKD) Using the Kidney Score (GFR)

In 2002, the National Kidney Foundation published treatment guidelines that identified five stages of CKD based on declining GFR measurements. The guidelines recommend different actions based on the stage of kidney disease.

Increased risk of CKD. A GFR of 90 or above is considered normal. Even with a normal GFR, you may be at increased risk for developing CKD if you have diabetes, high blood pressure, or a family history of kidney disease. The risk is increases in older individuals: People over 65 are more than twice as likely to develop CKD as people between the ages of 45 and 65. African Americans and Hispanics also have a higher risk of developing CKD.
Stage 1: Kidney damage with normal GFR (90% or above). Kidney damage may be detected before the GFR begins to decline. In this first stage of kidney disease, the goals of treatment are to slow the progression of CKD. If your kidney score is 90% or higher, a urinalysis may show you are in Stage 1.
Stage 2: Kidney damage with mild decrease in GFR (60% to 89%). When kidney function starts to decline, your health care provider will estimate the progression of your CKD and continue treatment to reduce the risk of other health problems.
Stage 3: Moderate decrease in GFR (30% to 59%). When CKD has advanced to this stage, anemia and bone problems become more common. Work with your health care provider to prevent or treat these complications.
Stage 4: Severe reduction in GFR (15% to 29%). Continue following the treatment for complications of CKD and learn as much as you can about the treatments for kidney failure. Each treatment requires preparation. If you choose hemodialysis, you will need to have a procedure to make a vein in your arm larger and stronger for repeated needle insertions. For peritoneal dialysis, you will need to have a catheter placed in your abdomen. Or you may want to ask family or friends to consider donating a kidney for transplantation before dialysis is needed.
Stage 5: Kidney failure (GFR less than 15%). When the kidneys do not work well enough to maintain life, you need dialysis or a kidney transplant.