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Kidney tests
Dr Asheesh Mehta Internal Medicine Specialist February 08, 2018
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The kidneys are vital organs serving multiple functions of which elimination of wastes and maintaining fluid, electrolyte and acid-base balance are the most important. They also produce hormones like erythropoietin and renin. The former stimulates bone marrow to produce red blood cells while the latter is important in maintaining fluid and electrolyte balance and in controlling blood pressure. The kidneys also activate the biologically inert form of vitamin D to its active form. The two kidneys are bean shaped organs located on either side of the spine in the upper abdomen. Each kidney is about 11.5 cm in length and weighs approximately 150 gm in the adult. The functional unit of the kidney is the nephron and there are about a million nephrons in each kidney. About 20% of the blood pumped out by the heart is delivered to the kidneys where it undergoes a complex process of ultrafiltration with the kidneys retaining required constituents and eliminating unwanted ones by a process of selective filtration, secretion and reabsorption. Healthy kidneys have a substantial spare capacity. Only about 40% of their capacity suffices for normal requirements. This spare capacity is why a person is able to donate a kidney for transplantation to another person since even a single normal kidney is easily able to meet requirements. Kidney function deteriorates due to a variety of reasons which are conveniently considered as pre-renal, renal and post-renal. Pre-renal conditions are those before the kidneys themselves and examples include severe fluid loss and dehydration due to severe diarrhoea and vomiting, bleeding or shock with marked fall in blood pressure from any cause. Renal conditions are those directly affecting the kidneys such as diabetes, damage due to glomerulonephritis or from heavy metals, etc. Post-renal conditions are those in which kidneys suffer damage because of back-pressure from obstruction to urine flow.

A number of tests are available to assess kidney function and structure. Since the kidneys have a lot of functions no single test is able to give complete information on the state of the kidneys. Urine examination is a simple test which provides quite a lot of indirect information about the kidneys. A number of parameters are analyzed in a urine examination. Presence of protein, usually albumin, is one of the important ones. Normally there is no protein in urine for the kidneys are extremely efficient in retaining all protein in the blood filtered by them. When nephrons are damaged, protein leaks into urine. Even small quantities are viewed with concern in diabetes as they portend early damage to nephrons which over time could progress to renal failure. Microalbumin assay is the test carried out in diabetics to detect this loss of minute quantities of protein. In some conditions massive protein leakage may result, enough to cause protein deficiency with associated swelling of the face and feet which is called nephrotic syndrome. Colour of the urine is also inspected with unusually yellow or orange colour suggesting greatly concentrated urine or presence of bile pigments or blood. Bile pigments in urine are indicative of jaundice due to liver disease while blood may be from kidney diseases such as stone or cancer or from diseases of the urinary bladder. There are many other causes of unusual colour of urine with medicines accounting for some of these cases. Microscopy of urine with focus on red blood cells and pus cells is very important. The former indicates presence of blood while the latter suggests infection in the urinary bladder or kidneys.

A number of tests are available to assess kidney function and structure. Since the kidneys have a lot of functions no single test is able to give complete information on the state of the kidneys
Information on the functional status of the kidneys is obtainable from a variety of tests. The glomerular filtration rate (GFR) is a measure of the flow of blood through the glomeruli of the nephrons. The GFR is an indirect measure of the amount of functioning kidney tissue. To get accurate measurements of GFR quite elaborate tests which are not generally practical for clinical use need to be done. In practice an idea of the GFR is obtained indirectly from tests such as blood urea and serum creatinine. The efficiency with which these naturally occurring substances are cleared from blood, giving an indirect idea of the GFR, is assessed by their levels in blood. These tests are not able to detect lesser degrees of damage to kidneys. Blood urea level is also affected quite a bit by factors such as dehydration. Serum creatinine is more reliable as a measure of renal function but it’s level is influenced by how much protein is consumed. Also, serum creatinine level starts rising only when kidney function is less than 50%. Even people with a single kidney will have a normal serum creatinine level. Thus, even a slight rise in serum creatinine level means that kidney function is compromised by more than 50%. Creatinine clearance is a better indicator of kidney function than is serum creatinine. It is a measure of the volume of blood cleared of creatinine in a particular time period. The conventional way of assessing creatinine clearance requires testing of serum creatinine as well as creatinine in a 24 hour sample of urine. This is obviously a tedious procedure with significant logistical problems. Formulae are available to calculate creatinine clearance from the serum creatinine (without need for urine collection and urinary creatinine estimation) along with other readily obtainable parameters such as age, sex, race, weight in a medically stable person. The estimated GFR derived from a calculation of the creatinine clearance suffices in most patients.

Since the kidneys have a major role in control of fluid, electrolyte and acid-base balance laboratory tests to assess these functions are part of any detailed investigation of the kidneys. Serum creatinine measurement serves as a screening test for serious kidney disease and these other tests are usually carried out only when creatinine level is high or when other specific indications are present. Measurements of serum sodium, potassium, chloride, calcium, magnesium, phosphorus and bicarbonate are some of the tests carried out in such cases. Serum potassium is particularly linked to a failing kidney and can be lethal if not corrected. Advanced renal failure causes a metabolic acidosis which is associated with a substantial fall in bicarbonate level which may need correction. Levels of the other electrolytes vary quite a bit depending on the stage of renal decompensation, underlying cause, associated complications, etc.

Imaging tests provide information about the size and structure of the kidneys. Among the more commonly available imaging tests are plain x-ray, ultrasound examination, intravenous pyelography (IVP) and CT scan. The plain x-ray has largely been superseded by ultrasound which gives good information about the size, presence of stones and whether any back-pressure changes from obstruction are occurring. IVP involves x-rays after administration of a contrast dye. The information obtained about the structure and function of the kidneys is quite good but injection of the dye may cause a reaction and sometimes these reactions are severe and occasionally fatal. CT scan, with or without contrast injection, is also very useful. Other specialized imaging tests are indicated in specific circumstances.

Kidney biopsy involves taking a small sample of kidney tissue for examination under the microscope after appropriate staining. Special immuno-histochemical stains may also be used and more specialized studies such as electron microscopy may also be carried out. The biopsy is usually obtained by means of a special needle which cuts a small bit of kidney tissue with minimal trauma. The needle is inserted through the skin. In earlier days the needle was introduced blindly and its entry into the kidneys was judged from indirect signs. Nowadays, the biopsy is usually performed under imaging guidance, usually ultrasound guidance. There is a small but nonetheless significant risk for complications such as bleeding and infection. Hence, before the procedure, bleeding disorders and uncontrolled high blood pressure need to be excluded. Only in occasional cases is an open surgical biopsy advised. A biopsy is most useful in conditions like unexplained excessive protein in the urine, unexplained renal failure, problems in renal function after a kidney transplant, suspected kidney cancer, etc.

It remains challenging to test the kidneys for early disease which is very difficult to detect from usual screening tests. Once kidney function has deteriorated substantially or once structural changes are well advanced, abnormalities are much more easily detectable but this is obviously not the stage at which it is desirable to first detect kidney disease. A judicious mix of early reporting of symptoms by the patient, a high index of suspicion on the part of the doctor along with good clinical skills and early and appropriate investigations may be successful in detecting kidney disease in its early stages when usually it is most responsive to treatment.
 

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