Creatinine Clinical Chemistry Study Notes

CREATININE IN CLINICAL CHEMISTRY

Overview of Creatinine

• Creatinine: A key renal function test used to detect kidney disease through serum and urine analysis.

  • Other tests used to evaluate kidney function include:

  • Urea Nitrogen (BUN)

  • Glomerular Filtration Rate (GFR)

  • Cystatin C

Formation and Elimination

• Source of Creatinine:

  • Formed from creatine and creatine phosphate in muscle tissue.

  • Excreted into plasma at a constant rate, proportional to muscle mass.

  • Reference Range: Varies with age and gender;

    • Males typically have higher creatinine levels due to greater muscle mass.

• Clearance Measurement:

  • Creatinine clearance from plasma by the kidneys is a measure of GFR.

  • Relationship to GFR:

  • Plasma creatinine is inversely related to GFR.

  • As GFR decreases, plasma creatinine levels increase, indicating reduced renal function.

Clinical Significance of Plasma Creatinine

• What Plasma Creatinine Reflects:

  • Muscle Mass: Relatively stable over time.

  • Rate of Creatine Turnover: Stable.

  • Renal Function.

• Increased Serum Creatinine:

  • Indicates decreased renal function (lowered GFR).

  • May not elevate until kidney function is reduced by 50%:

    • For every 50% reduction in GFR, creatinine levels double.

• Decreased Serum Creatinine:

  • Reflects decreased muscle mass or muscle atrophy.

  • Positive pregnancy effect: Increased GFR leads to lower serum creatinine concentrations.

Creatinine Measurement Techniques

• Purpose of Measurement:

  • Assess kidney function (not sensitive enough to detect mild dysfunction).

  • Determine the severity of kidney disease.

  • Monitor progression of kidney disease.

• Urine Creatinine:

  • Typically stable on a day-to-day basis, making it useful for verifying completeness of 24-hour urine samples.

  • Other urine constituents can be quantified as a ratio to creatinine (e.g., albumin/creatinine ratio in random urine).

Measurement Variables

• Types of Samples:

  • Plasma, serum, urine

  • Urine samples should be refrigerated if testing is delayed.

• General Interferences:

  • Hemolysis: Affects Jaffe method.

  • Icterus (jaundice): Introduces negative bias.

  • Lipemia: Affects several methods; inhibits optical readings.

Assay Methods for Creatinine

• Jaffe Reaction:

  • Utilizes alkaline picrate to form a red-orange chromagen.

  • Non-specific method: Numerous interferences.

  • Can be performed as a kinetic reaction, which has fewer interferences compared to endpoint reactions.

• Enzymatic Methods:

  • Involve coupled reactions.

• Dynamic Range for Measurement:

  • Serum/Plasma: 8.84 – 2210 µmol/L

  • Urine: 0.88 – 35.36 mmol/L

  • For samples exceeding the high end of the analytical range, dilute with saline and re-analyze, then multiply results by two.

Endpoint vs Kinetic Reactions

• End-Point Assays:

  • Measure absorbance at completion.

  • Can use:

  • Single standard,

  • Standard curve,

  • Molar absorptivity methods for concentration calculation.

  • Enzyme analyses not performed as end-point reactions.

• Kinetic Reactions:

  • Do not reach completion; absorbances are measured at intervals over time.

• Phases in Reactions:

  • Lag Phase: Reactants begin to interact.

  • Reacting Phase: Product formation occurs.

  • Reagent Depletion Phase: Reactants dwindle as the reaction progresses.

Analytical Methods for Creatinine (Table 12-6 Summary)

• Chemical Methods Based on Jaffe Reaction:

  • Jaffe reaction: creatinine reacts with picrate to form a colored complex.

  • Jaffe-kinetic: optimized to avoid interference of non-creatinine chromogens.

  • Jaffe methods with and without adsorbents to improve specificity.

• Enzymatic Methods:

  • Involve steps that convert creatinine into other bi-products measured by changes in absorbance.

• Other Methods:

  • Isotope dilution mass spectrometry is a highly specific reference method for quantification.

Beckman Creatinine Methodology

• Involves introducing a precise volume of sample into the Jaffe reagent.

• Detectors measure absorbance increases at 520 nm and 560 nm, with timing changes to determine creatinine concentration.

Calibration Procedures

• Calibration Ranges:

  • Synchronization AQUA Cal 1 & 2: Must be calibrated every 48 hours or following maintenance/troubleshooting.

• DXC Calibration:

  • Involves checks for accuracy, precision, and sensitivity with previous standards thrown out for highest and lowest readings.

  • Acceptability Parameters: Focus on range, accuracy, precision (back-to-back), and span sensitivity.

Quality Control in Measurements

• Running Quality Controls:

  • At least one set per shift, with special checks post maintenance or lot changes.

• Common QC Issues:

  • Improper temperature storage for control samples can cause failures.

  • Errors must be identified as calibration issues or QC only concerns.

Sources of Error in Creatinine Assays

• Different components can bias results, affecting accuracy:

  • Jaffe Assay: Affected by ascorbate, uric acid, glucose, ketones, and cephalosporins.

  • Enzymatic Assays: Affected by ascorbate (when associated with peroxidase) and certain drugs like dopamine and lidocaine.

Reference Ranges for Creatinine

• Serum/Plasma:

  • Children: 27-62 µmol/L

  • Adults: Males 80-115 µmol/L; Females 53-97 µmol/L.

• Critical Levels: Values greater than 600 µmol/L are deemed critical.

• 24-hour Urine Excretion:

  • Males: 7.1-17.7 mmol/day

  • Females: 5.3-15.9 mmol/day

  • Random urine does not have a standardized reference range.

References

• Beckman Coulter Instructions For Use: UniCel® DXC Synchron Clinical Systems, Volume 1, 2008.

• Beckman Coulter In-Lab Training Manual: UniCel® DXC 800, 2005.

• Clinical Chemistry: Principles, Techniques, and Correlations, 8th Edition. Bishop, M.L.; Fody, E.P.; Schoeff, Wolters Kluwer 2018.