Nonprotein+Nitrogen+Compounds

Nonprotein Nitrogen Compounds

  • Nonprotein nitrogen compounds (NPN) are biochemical entities important in assessing renal health.

Monitoring Renal Function

  • NPN is used to monitor renal function through the measurement of:

    • Creatinine

    • Creatine

    • Urea

    • Uric acid

    • Ammonia

  • These compounds are products of protein and nucleic acid catabolism.

Clinically Significant Nonprotein Nitrogen Compounds

  • Approximate plasma concentration of total NPN:

    • Urea: 45-50%

    • Amino acids: 25%

    • Uric acid: 10%

    • Creatinine: 5%

    • Creatine: 1-2%

    • Ammonia: 0.2%

Urea

  • Urea is the NPN present in the highest concentration in blood and is a major excretory product of protein metabolism.

    • Derived from:

      • Proteins (Proteolysis, primarily enzymatic)

      • Amino acids (Transamination and oxidative deamination)

      • Ammonia (Synthesis in the urea cycle)

Urea Synthesis and Excretion

  • Synthesized in the liver from nitrogen released during amino acid metabolism.

  • Excreted primarily by the kidneys, with some loss through gastrointestinal tract and skin.

  • Plasma urea concentration can be affected by:

    • Diet protein content

    • Rate of protein catabolism

    • Renal function and perfusion

Clinical Applications of Urea Measurement

  • Evaluation of renal function.

  • Assessment of hydration status.

  • Determination of nitrogen balance.

  • Aid in the diagnosis of renal disease.

  • Verify the adequacy of dialysis.

Measurement of Urea

  • Urea measurements were originally performed on a protein-free filtrate of blood, reporting nitrogen concentration.

  • To convert urea nitrogen to urea concentration, multiply by 2.14.

  • To convert urea from mg/dl to mmol/L, multiply by 0.36.

Analytical Methods for Urea

  • Reaction:

    • Urea + 2H2O --> Urease --> 2NH4 + CO3

    • NH4 + 2-oxoglutarate (GLDH) --> glutamate + H2O + NADH + H+ --> NAD+

  • GLDH (Glutamate dehydrogenase) is a key enzyme in the process.

Specimen Requirements for Urea Measurement

  • Urea concentration may be measured in plasma, serum, or urine.

  • Avoid sodium citrate and sodium fluoride anticoagulants.

  • Fasting is not required; non-hemolyzed samples recommended.

  • Specimens that cannot be immediately analyzed should be refrigerated.

Reference Intervals for Urea

  • Plasma/serum: 6-20 mg/dl (2.1-7.1 mmol/L)

  • 24 hr. urine: 12-20 g/d (0.43-0.71 mol urea/d)

  • Plasma concentrations may vary:

    • Slightly lower in children/pregnancy

    • Higher in males versus females

  • In untreated ESRD, plasma urea may reach 108 to 135 mg/dL (40 to 50 mmol/L).

  • Decreased urea levels may indicate low protein intake or severe liver disease.

Pathophysiology of Urea Levels

  • Pre-renal azotemia:

    • Reduced renal blood flow; increased blood urea (e.g., CHF, shock).

  • Renal azotemia:

    • Decreased renal function (acute/chronic renal failure).

  • Post-renal azotemia:

    • Urinary tract obstructions causing high urea and creatinine levels.

Creatinine

  • Creatinine is produced from creatine in muscles and is an indicator of renal function.

  • Created at a constant rate by muscle mass, primarily removed via glomerular filtration.

  • Not secreted or absorbed by renal tubules.

  • Plasma creatinine concentration is a stable measure used for renal function evaluation.

Creatinine Disease Correlations

  • Increased plasma creatinine indicates decreased glomerular filtration.

  • May remain stable until renal function is decreased by approximately 50%.

  • Regular checks required for abnormal changes.

Analytical Techniques for Creatinine Measurement

  • Classic Jaffee Method:

    • Creatinine + Picrate Acid --> Colored chromogen.

    • Plasma or serum specimen required.

Uric Acid

  • Breakdown product of purines; elevated levels can indicate gout and kidney stones.

  • Analytical Method:

    • Uric acid + O2 + H2O --> Allantoin + CO2 + H2O2.

Ammonia

  • Produced from amino acid deamination; highly toxic, converted to urea in the liver.

    • Increased levels may indicate liver failure, Reye's disease.

Creatinine Clearance

  • Calculated to measure how effectively creatinine is removed from plasma by kidneys.

  • Formula:[Creatinine Clearance = (Creatine concentration of 24-hour urine) / (Plasma creatinine concentration)]

Reference Ranges of Renal Function Tests

  • BUN: 10 - 20 mg/dl

  • Creatinine: 0.5 - 1.5 mg/dl

  • Uric Acid: 3.0 - 7.0 mg/dl

  • Creatinine Clearance: 90 - 130 ml/min

  • Ammonia: 20 - 60 µg/dl

Functions of Kidneys

  • Maintenance of extracellular fluid volume/composition.

  • Excretion of metabolic waste products.

  • Regulation of blood pressure.

  • Synthesis of erythropoietin and production of vitamin D3.

Factors Affecting Renal Function

  • Pre-renal conditions (e.g., shock, dehydration).

  • Post-renal conditions (e.g., obstruction to urinary outflow).

Indications for Renal Function Tests

  • Early identification of renal impairment.

  • Diagnosis and monitoring of renal disease.

  • Adjust drug doses based on renal function.

Conditions with Increased Risk for Chronic Renal Disease

  • Diabetes Mellitus, Hypertension, Autoimmune diseases, Older age, Family history of renal disease.

Tests to Evaluate Glomerular Function

  • Clearance tests to measure GFR, including 24-hour urine collections.

Significance of GFR

  • The GFR indicates the rate of substances cleared from circulation by glomeruli, with normal values being 120-130 ml/min/1.73 m².

Prediction Equations for GFR

  • Cockcroft-Gault:[C CrCl = (140-age) * weight (kg)/(72 * S.Cr)]

    • Females: multiply by 0.85.

  • MDRD Formula for estimating GFR with different variables considered.

Chronic Kidney Disease (CKD)

  • Gradual decline in renal function often related to age, hypertension, and diabetes.

Complications of Renal Biopsy

  • Risks can include hemorrhage, infection, arteriovenous fistula, rare perforation.

Cystatin C and β2 microglobulin

  • Cystatin C is a sensitive indicator of early kidney impairment.

  • β2 microglobulin indicates increased cellular turnover and may elevate in renal failure.

Microalbumin and Neutrophil Gelatinase-Associated Lipocalin (NGAL)

  • Microalbumin presence signals early kidney damage; NGAL is a fast marker for acute kidney injury and correlates with prognosis.

Glomerular Diseases

  • Includes acute nephritis, chronic glomerulonephritis, and nephrotic syndrome, all with specific laboratory findings.

Tubular Diseases

  • Tubular function impairment can occur in various renal diseases; interstitial nephritis is a notable condition.

Renal Calculi and AKI

  • Kidney stones may recur and are treated based on the underlying cause. AKI classifications based on causative factors include pre-renal, intrinsic, and post-renal.

Evaluation of Renal Function in Clinical Practice

  • Regular assessment and care for patients, especially at risk for renal impairment.