Kidney Function Tests.pdf

Clinical Pathology Dr. Wisam Salim Hassan Kidney Function: The kidneys are the chief organs regulating the internal environment of the body. They have important functions such as: 1. Elimination of excess water formed or introduced into the body 2. Elimination of inorganic elements accord...

Clinical Pathology Dr. Wisam Salim Hassan Kidney Function: The kidneys are the chief organs regulating the internal environment of the body. They have important functions such as: 1. Elimination of excess water formed or introduced into the body 2. Elimination of inorganic elements according to the needs of the body 3. Elimination of nonvolatile end products of metabolic activity 4. Retention within the body of substances required for the maintenance of normal functions 5. Elimination of certain foreign toxic elements 6. Formation and excretion of substances such as hydrogen ions and ammonia Renal Function Tests: These include: 1- Urine specific gravity and the effect of water deprivation 2- Estimation of non-protein nitrogen2 level in the blood 3- Studies of the ability of the kidney to excrete certain dyes 4- Tests based on the clearance concept 5- Other blood chemistry determinations, such as serum Amylase, electrolytes, blood pH, cholesterol and serum proteins Nonprotein Nitrogen Levels in Blood: The term nonprotein nitrogen (NPN) is used to identify nitrogen containing components of serum or plasma that are not associated with protein. This group consists of a heterogeneous mixture of substances not precipitated by commonly used protein precipitants. Nonprotein nitrogens include: Urea, creatinine, creatine, uric acid, ammonia, amino acids, and a fraction designated “ undetermined nitrogen”. The NPN substances represent products of intermediary metabolism of both tissue and ingested protein. Clinical Pathology Dr. Wisam Salim Hassan A- Blood Urea Nitrogen (BUN) Urea is formed in the liver and represents the principal end product of protein catabolism. This substance normally has no useful function in the body other than a possible mild diuretic action and is excreted almost entirely by the kidneys. The glomerulus filters urea in plasma and under normal conditions approximately 25 to 40% of filtered urea is reabsorbed as it passes through the tubules. Indications for Blood Urea Nitrogen Testing: 1. Whenever decreased kidney function is suspected 2. As a technique for measuring peripheral perfusion of tissues in animals subject to hypovolemic shock or decreased blood pressure 3. As a routine presurgical laboratory screening test Technique of Tests: Blood Urea nitrogen determinations usually conducted on whole blood, plasma , and serum. Anticoagulants containing any nitrogen must be avoided. The anticoagulant of choice is ethylenediaminetetraacetic acid (EDTA), although heparin, sodium citrate, and sodium oxalate are acceptable. A variety of laboratory techniques have been developed for estimating blood urea nitrogen concentration such as: 1. Chromatography technique 2. Mercury combining power 3. Nesslerization 4. Determination of Urease enzyme Chromatography technique Advantages of this method are include: Its simplicity, no special chemicals are necessary, and the small amount of serum or plasma required. Clinical Pathology Dr. Wisam Salim Hassan Principle of the test The chromatography paper is banded with different reagents. When the tip of the paper is placed in the sample, there is an upward migration so that the serum passes through a series of chemical reactions. This produces a color change the height of which is directly related to urea nitrogen concentrations. Mercury combining power Mercury will combine with chemicals such as urea, creatinine, and uric acid when a mercuric salt solution is added to substances containing these products. A protein-free filtrate is prepared and mercuric chloride is added to it. When mercuric chloride has ceased combining with nonprotein nitrogen substances in the filtrate, it will combine with the test reagent sodium carbonate to produce a reddish brown precipitate. Nesslerization This method is conducted by adding urease enzyme to a serum sample, and this will lead to release of ammonia. After that, Nessler reagent is added to the mixture which produces yellow color. Then, blood urea nitrogen can be determined colorimetrically with spectrophotometer.
Interpretations of the results of laboratory tests for blood urea nitrogen: 1. If BUN concentration exceeds 35 to 45 mg/dl, GFR is diminished 2. Abnormal BUN concentration known to be caused by abnormal excretion may be due to prerenal, primary renal, or postrenal factors. 3. Correlation can be made between the degree of elevation of BUN and the severity of renal function impairment. Clinical Pathology Dr. Wisam Salim Hassan B- Creatinine It is a nonprotein nitrogen substance formed during muscle metabolism of creatine and phosphocreatin. It is excreted by glomerular filtration. The measurement of serum creatinine may provide an index of glomerular filtration. Most factors influencing creatinine concentration are similar to those influencing blood urea nitrogen levels with the following exceptions: 1. Creatinine is not influenced by diet 2. Daily production of creatinine from muscle metabolism is relatively constant 3. Creatinine production is not as easily influenced by catabolic factors affecting urea formation. Therefore, conditions such as fever, toxemia, infection, and drug administration do not readily influence creatinine levels. The rate of excretion is influenced by glomerular filtration rate (GFR), and any abnormality that decreases GFR will result in an increase in the concentration of serum creatinine. The use of Lloyd’s reagent is recommended to determine the serum levels of creatinine.
Interpretations of creatinine concentration is similar to that for blood urea nitrogen level.