The kidneys are the main organs in the body responsible for filtering waste products out of the bloodstream. They are responsible for filtering out the waste that is not needed and reabsorbing the chemicals and water that can still be utilized by the body. On a daily basis the kidneys process approximately two hundred quarts of blood and filter out two quarts of waste material which becomes urine. If we did not have the kidneys to remove these waste products from the blood they would quickly build up and begin to severely damage the body.
The actual point where filtration occurs in the kidney is in units called nephrons. Inside the millions of nephrons in each kidney rest a glomerulus, which is a tiny cluster of blood vessels surrounding a urine collection tubule. The capillaries of the glomerulus are very permeable allowing wastes and water to pass into the urine tubules while keeping proteins and blood in the bloodstream. Most diseases of the kidneys attack this portion of the kidney. A healthy person has 100% kidney function and most people could live with 30-40% kidney function without noticing any ill health effects. Serious effects on health occur when kidney function falls below 25% and end stage renal disease is classified as renal function below 15%.
Kidney disease is caused by damage to either the vasculature inside the nephron or to the structure inside the organ. The two most common causes of chronic kidney disease are Diabetes and Hypertension. Diabetes causes glucose to build up in the blood and poison the kidney. Unused glucose gets filtered out by the kidney causing damage to the vessels in the glomeruli over time. Hypertension causes damage to the vessels in the glomeruli just as it causes damage to all the blood vessels in the body. In both cases, as the vessels are damaged the walls thicken and the nephrons can no longer filter wastes at full capacity. Other diseases attack the glomeruli such as infections, sclerotic diseases, and autoimmune diseases. Once again, the vessels harden up with damage and cannot filter blood properly. Polycystic kidney disease is a genetic disorder where cysts grow inside the kidney causing structural damage. The cysts grow and eventually replace the mass of the kidney causing kidney failure. Drugs and poisons can cause kidney damage by damaging the glomeruli. Physical trauma can cause structural damage as can obstructions like kidney stones and tumors. Even something like an enlarged prostate gland in men can cause kidney damage. Whether kidney failure is acute (AKF) or chronic (CKF) doctors rely on laboratory tests to help determine on how to diagnose and treat the patient.
There is no one test that can assess kidney function. The clinician uses a variety of tests on both serum and urine to assess a patient’s level of kidney function. The most commonly known test is serum creatinine. Creatinine is a waste product of various bodily processes, especially muscle activity. It is carried by the bloodstream to the kidneys where it is freely filtered by the glomerulus. Some creatinine gets reabsorbed by the tubules and reused by the body. Normal levels are age dependent and can range anywhere between 0.2-1.3 mg/dL. An elevation in serum creatinine is usually a sign that the kidneys are not functioning properly. However estimating kidney function based on old creatinine kidney function formulas is not entirely accurate. The Glomerular Filtration Rate (GFR) calculation was developed to increase the accuracy of accessing kidney function. The calculation is based on values for serum creatinine, age, gender, and race. A true Gomerular Filtration Rate test measures glomerular filtration by injecting the patient with a tracable form of insulin, collecting a twenty-four hour urine, and then testing the urine for the analyte to see how much is recovered. Based on this original test the estimated GFR calculation was developed. The calculation is a cost effective solution to collecting twenty-four hour urines and still measures the functioning renal mass and filtering capacity of the kidneys. GFR’s are said to be “normalized” for body surface area. Normalizing allows for the calculated GFR to be run on people of various body sizes by setting the body surface area (BSA) value in the equation at 1.73m2. Normal ranges can vary from lab to lab. In general a normal range for the GFR is >60 ml/min/1.73m2 for both African Americans (GRFAA) and non- African Americans (GFRNA). In general when assessing chronic kidney disease GFR values between 30 and 59 are indicative a moderate decrease in kidney function. GFR values between 15 and 29 are indicative of a severe reduction of kidney function, and a GFR less than 15 is labeled as kidney failure.
Another test to measure functioning renal mass and filtering capacity of the kidneys is the Creatinine Clearance. This test is also performed on at twenty-four hour urine collection and it approximates the rate that creatinine is filtered out by the glomeruli of the kidneys. However because creatinine is reabsorbed by the tubules this test is not as accurate as the GFR at assessing renal function. A generalized normal range for creatinine clearance is 88-128 ml/min. For a random urine creatinine the range is around 30-125 mg/dL and for a twenty-four hour urine creatine the range is around 600-2500mg/24hr. Checking for urine creatinine becomes more valuable when it can be compared with a urine protein.
Kidney disease is usually first detected by the presence of persistent proteinuria. Random spot checks for protein are done with every urinalysis. When a urinalysis comes back positive for protein a clinician will usually order a urine creatinine, a urine protein, and possibly a microalbumin. These three tests can help determine what level of kidney disease, if any, a patient is experiencing. Healthy kidneys do not release protein into the urine. When a glomerulus becomes damaged proteins can leak through causing even more damage. In early stages of kidney disease when damage is small only small proteins will leak through such as albumin. Once this occurs the patient is said to have microalbuminuria. This is usually seen in patients such as diabetics. The clinician will order a urine microalbumin level. Most labs are actually running an albumin to creatinine ratio when they run a microalbumin level. This ratio compares the two values to determine the level of kidney function of the patient. A generalized normal range for a random microalbumin is <30 mg Mcalb/g Creat. Microalbumin testing is also used to continuously monitor patients, such as diabetics, who are already known to have kidney disease. Because microalbumin testing is looking for such small amounts of protein one should take note that certain conditions can give false positives. Uncontrolled diabetes, uncontrolled hypertension, menstrual contamination, infection, and even strenuous exercise can elevate microalbumin results. These conditions should be taken into consideration before testing the patient. Once the glomeruli become moderately to severely damaged, larger proteins begin to pass through into the urine. The patient is said to have proteinuria. At this point there is no need to test just for small amounts of albumin. A urine total protein will test for all types of protein that are leaking into the urine. A generalized normal range for a random total urine protein is <11.9 mg/dL and the range for a twenty-four hour total urine protein is <149.1 mg/24hr. There is merit to running twenty-four hour urines in that they offer more accuracy in measurement than random urine testing. If the clinician wants to know what types of protein are in the urine, samples can be sent out for chromatographic and electrophoretic testing. The benefit of knowing what types of proteins are in the urine can help out in instances of autoimmune and hereditary kidney diseases. Once testing has confirmed the presence of kidney damage the clinician can take the best course of action to ensure that remaining kidney function is stabilized.
Even though a person can live a normal life with a significant reduction in kidney function, kidney damage is irreversible. Persons with known chronic diseases such as diabetes and hypertension should take care to have their clinician monitor their kidney function. The easiest way to detect kidney problems is with a spot urine protein test, a serum creatinine, and a calculated GFR. Diabetics should have their microalbumin/creatinine ratios checked yearly to monitor for the onset of kidney disease. For everyone the best way to ensure that the kidneys stay healthy is to control glucose levels, blood pressure, cholesterol levels, and not smoke. Having healthy blood vessels will ensure that you have a healthy heart and healthy kidneys.
* Burtis, C. A., Ashwood, E. R., Bruns, D. E., Tietz Textbook of Clinical Chemistry & Molecular Diagnostics. St. Louis, Missouri: Elsevier (2006). p.797-815.
* The Kidneys and How They Work. (2009).National Kidney and Urologic Diseases Information Clearinghouse: The Kidneys and How They Work. Retrieved on November 28, 2009 from http://kidney.niddk.nih.gov/Kudiseases/pubs/yourkidneys/
* FAQs About the Glomerular Filtration Rate. (2003). The IHS Provider: FAQs About the Glomerular Filtration Rate. Retrieved on November 28, 2009 from http://www.ihs.gov/medicalprograms/kidney/pro_clinicaltools/FAQsaboutGFR.pdf.
* Chronic Kidney Disease. (2009). National Kidney Foundation: Chronic Kidney Disease. Retrieved on November 28, 2009 from http://www.kidney.org/kidneydisease/ckd/index/cfm.