Patient Education: Testing for kidney disease.

Physiology of Nephron (Photo credit: Wikipedia)

The kidney is a highly selective filter than receives a large supply of blood from the circulatory system. The entire blood volume passes through the kidney several times over per day. The kidney selectively removes toxins from the blood and in the process forms urine which leaves the body through the ureters bladder and urethra.

The Ultimate goal of the diverse functions of the kidney is to preserve a careful balance of electrolytes minerals and water in order to maintain the finely tuned processes which give rise to life. The ultimate result of kidney failure is break down of the myriad pathways which rely on this balance. Every organ system therefore relies to some extent on the daily functions of the kidney and the kidney relies on feedback from these systems to determine how well its doing its job. When the system works as it ought to there is a perfect balance between intake of nutrient and excretion of waste products.

The functions of the kidney are still being determined by scientists everyday. What has been learned so far is as follows:-

• The kidney detects the level of oxygen in the blood and stimulates the bone marrow to produce oxygen carrying blood cells as necessary to most efficiently maintain the oxygen level of the blood. It does this by producing a hormone known as erythropoeitin.

• The kidney receives and contributes critical information to important parts of the brain which determine if there is enough fluid within the body. If fluid is deemed to be in excess urine volume is increased and water is excreted to return the body to a state of fluid balance. If the body is dehydrated the kidney conserves water by decreasing urine formation to the minimum required to still excrete wastes.

• The kidney removes waste products by allowing them to filter into the urine and sometimes actively excreting them into the urine.

• The kidney constantly monitors urine that is being formed and reabsorbs from the urine anything that is still useful before it leaves the body.

• The kidney senses the flow through it and determines and instructs centers in the brain to increase or decrease blood pressure to maintain an appropriate flow rate to perform its function.

• The kidney removes acid from the body and is the most important source by which acidic compounds generated by daily activity is removed.

• The kidney plays an important role in bone formation as it is the last step in the synthesis of vitamin D.

Any test of kidney function should be able to to test individually these functions and determine if there is kidney disease present or not.

Some of these tests are as follows:-

• The ability of the kidney to filter is expressed by the term GFR or glomerular filtration rate. This is an index of all the fluid passing across the filter per minute of every day. This number can be determined by a series of calculations performed using the ratios of concentrations of various substances in the blood vs the urine.

1. This may be measured by a 24 hour collection of all urine passed. The GFR being calculated from the ratio of creatinine in the urine and blood.
2. A radioactive material may be injected and the rate of the excretion of the radioactive material in the urine determined using a collector or counter for radiation. This is termed a renal scan.
3. The concentration of a substance that is generated within the body, that we know should be promptly excreted by the kidney could be measured with a single blood test. This is the basis of the measurement of creatinine and blood urea nitrogen. When the kidney is functioning normally it should be within the normal range. Unfortunately these tests are not as accurate as the above more direct methods of determining GFR. They should really be considered markers of function vs dysfunction.
4. The GFR may be calculated based on a single test for creatinine by using a formula that takes into account the age, weight, gender and race of the subject. This is more reliable that relying on the BUN or creatinine alone.

• The ability of the kidney to detect and respond to the oxygen carrying content of the blood is determined by measuring the blood count on a routine blood test. But there are many causes for a low blood count. Only after excluding all the most common causes can we directly say that the kidney is the reason why the blood count is low and thus prompt a more thorough work up for kidney disease

• Regularly checking the blood pressure is a good way to detect kidney disease. As one of the functions of the kidney is blood pressure regulation patients with high blood pressure should have one of the above tests of kidney function done.

• Testing the urine for protein blood and any abnormal constituent of urine. There are substances that do not enter urine under normal circumstances because the kidney reabsorbs them during the process of forming urine or selectively does not allow the them to be filtered in the first place. The most important of these is protein, persistent levels of protein in the urine may imply damage to the filtration apparatus of the kidney. This is one area where early intervention is very important. This is why a dipstick of the urine is a standard procedure when you see your physician.

• Testing the level of acid in the blood or urine may draw attention to a previously unknown kidney disease.

• Low vitamin D levels or low calcium levels may be due to kidney disease.

• Testing the level of various electrolytes whose concentrations are determined by kidney function may also highlight previously unknown kidney disease