By Alan L. Rubin

Your kidneys rid your body of many harmful chemicals and other compounds produced during the process of normal metabolism. Your kidneys act like a filter through which your blood pours, trapping the waste and sending it out in your urine while the normal contents of the blood go back into your bloodstream. They also regulate the salt and water content of your body. When kidney disease (also known as nephropathy) causes your kidneys to fail, you must either use artificial means, called dialysis, to cleanse your blood and control the salt and water or receive a new working donor kidney, called a transplant.

Chronic kidney disease is more prevalent now than it has been in the past, and the major source of all these new cases is diabetes. In the United States today, half the patients who require long-term dialysis require it because of diabetes. Fortunately, the number requiring dialysis is on the decline because of the increasing awareness among people that they need to control their blood glucose. The incidence of kidney disease is only about 5 percent among people with type 2 diabetes, compared to 30 percent among people with type 1 diabetes; however, the absolute number of patients with kidney disease is about the same for the two groups because type 2 diabetes is so much more common than type 1.

The following sections tell you what you need to know to prevent and manage diabetic kidney disease — how diabetes affects your kidneys, what changes are occurring in your body, and how you can both check for them while they are still reversible and prevent them from getting any worse.

The impact of diabetes on your kidneys

Each kidney consists of about a million units called nephrons. Each nephron contains a structure called the glomerulus (the plural is glomeruli) that filters blood and separates out waste products and some water.

When you first get diabetes, your kidneys are enlarged and seem to function abnormally well, judging by how fast they clear wastes from your body. Your kidneys seem to function so well because you have a large amount of glucose entering your kidneys, which draws a lot of water with it and causes an increase in the pressure inside each glomerulus. This more-rapid transit of blood through the kidneys is known as an increased glomerular filtration rate. Early in the development of your diabetes, the membrane surrounding your glomeruli, called the glomerular basement membrane, thickens, as do other adjacent structures. These expanding membranes and structures begin to take up the space occupied by the capillaries inside the glomeruli, so the capillaries are unable to filter as much blood.

Fortunately, you have many more glomeruli than you really need. In fact, you can lose the equivalent of a whole kidney (half of each kidney) and still have plenty of reserve to clean your blood. If your kidney disease goes undetected for about 15 years, damage may become so severe that your blood shows measurable signs of the beginning of kidney failure, called azotemia. If the neglect of the disease reaches 20 years, both kidneys may fail entirely.

Not every person with diabetes is at equal risk for kidney disease and kidney failure. This complication seems to be more common in certain families and among certain racial groups, especially African Americans, Mexican Americans, and Native Americans. It is certainly more common when high blood pressure is present. Although doctors and researchers believe that high blood glucose is the major factor leading to nephropathy, only half of the people whose blood glucose has been poorly controlled go on to develop nephropathy.

Early indications of kidney disease

A healthy kidney permits only a tiny amount of albumin, a protein in the blood, to enter the urine. However, a kidney being damaged by nephropathy is unable to hold back as much albumin, and the level in the urine increases, causing microalbuminuria (the presence of tiny but abnormally high amounts of albumin in your urine). If your kidneys are on their way to being damaged by diabetic nephropathy (kidney disease caused by diabetes), doctors can detect microalbuminuria in your urine.

For three-quarters of the patients in the early stages of kidney disease, however, the amount of albumin in your urine is so small that it won’t trigger a positive result when the traditional urine dipstick test is used. Therefore, your doctor should perform a more sophisticated test for microalbuminuria. With the test, you collect a 24-hour urine specimen (meaning you save all the urine you produce in 24 hours and have it tested), by taking a random urine sample or by collecting a specimen over a certain time period, usually four hours. If the level of albumin is abnormally high, it needs to be checked once again to be certain, because some factors (such as exercise) can trigger a false positive test. A second positive test should lead to action to protect your kidneys.

Because microalbuminuria can be detected about five years before a urine dipstick would test positive for albumin, you have time to treat the onset. Furthermore, treatment during the stage of microalbuminuria can reverse the kidney disease. After macroalbuminuria is found (indicating much larger amounts of protein in the urine) using the dipstick method, the disease can be slowed but not stopped.

If you have had type 1 diabetes for five years or more, or if you’ve recently been diagnosed with type 2 diabetes, your doctor must check for microalbuminuria unless you’ve already tested positive for albumin with a urine dipstick. If your test comes back negative, you should have it rechecked annually.

As many as 25 percent of patients with no microalbuminuria still have kidney disease, so treatment with drugs that protect your kidneys makes sense.

In June 2003 in the New England Journal of Medicine, researchers showed that microalbuminuria doesn’t always lead to kidney failure. Patients with type 1 diabetes who improved their blood glucose levels, blood pressure, and abnormal blood fats experienced a decline in microalbuminuria and, therefore, a decline in kidney damage.