Clinical Pathology: Kidney Function

Kidney Function

• The kidneys regulate the internal environment of the body through various functions, including:
  • Eliminating excess water and inorganic elements
  • Removing nonvolatile end products of metabolic activity
  • Retaining essential substances for normal bodily functions
  • Eliminating foreign toxic elements
  • Forming and excreting substances like hydrogen ions and ammonia

Renal Function Tests

• These tests assess kidney function and include:
  • Urine specific gravity and water deprivation tests
  • Estimation of non-protein nitrogen levels in the blood
  • Studies on the ability of the kidney to excrete certain dyes
  • Clearance concept tests
  • Other blood chemistry determinations (e.g., serum amylase, electrolytes, blood pH, cholesterol, and serum proteins)

Non-Protein Nitrogen Levels in Blood

• Non-protein nitrogen (NPN) refers to nitrogen-containing components in serum or plasma not associated with protein
• NPN includes:
  • Urea, creatinine, creatine, uric acid, ammonia, amino acids, and "undetermined nitrogen"
• NPN represents products of intermediary metabolism from both tissue and ingested protein

Blood Urea Nitrogen (BUN)

• Urea is the principal end product of protein catabolism, formed in the liver and excreted primarily by the kidneys
• Approximately 25-40% of filtered urea is reabsorbed in the tubules
• Indications for BUN testing:
  • Suspected decreased kidney function
  • Measuring peripheral perfusion of tissues in animals with hypovolemic shock or decreased blood pressure
  • Routine presurgical laboratory screening

Technique of BUN Tests

• BUN determinations are usually conducted on whole blood, plasma, or serum
• Anticoagulants containing nitrogen must be avoided; EDTA, heparin, sodium citrate, and sodium oxalate are acceptable
• Various laboratory techniques are used to estimate BUN concentration, including:
  • Chromatography
  • Mercury combining power
  • Nesslerization
  • Determination of Urease enzyme

Chromatography Technique

• Advantages:
  • Simplicity
  • No special chemicals required
  • Small amount of serum or plasma needed
• Principle of the test:
  • Chromatography paper is banded with different reagents, which react with the serum to produce a color change, indicating urea nitrogen concentrations

Interpretations of BUN Results

• If BUN concentration exceeds 35-45 mg/dl, glomerular filtration rate (GFR) is diminished
• Abnormal BUN concentrations may be caused by prerenal, primary renal, or postrenal factors
• Correlation can be made between the degree of BUN elevation and the severity of renal function impairment

Creatinine

• Creatinine is a nonprotein nitrogen substance formed during muscle metabolism of creatine and phosphocreatin
• It is excreted by glomerular filtration and provides an index of glomerular filtration
• Factors influencing creatinine concentration are similar to those for BUN, except:
  • Creatinine is not influenced by diet
  • Daily production of creatinine from muscle metabolism is relatively constant
  • Creatinine production is not easily influenced by catabolic factors affecting urea formation

Interpretations of Creatinine Concentration

• Similar to BUN interpretations, with elevations indicating decreased GFR and potential renal function impairment

This quiz covers the functions of the kidneys in regulating the internal environment of the body, including elimination of excess water and waste products, retention of essential substances, and more.