Procedures

Understanding the principles and procedures for testing proteins and nitrogen compounds is key to clinical diagnosis. Proper technique and careful attention to detail will produce accurate and reliable data

Principles of Laboratory Tests

Tests measure the levels of proteins and nitrogen-containing compounds in bodily fluids, using a variety of chemical and immunochemical techniques

• Total Protein: Biuret reaction (measures peptide bonds)
• Albumin: Dye-binding methods (Bromcresol Green or Bromcresol Purple)
• Prealbumin: Immunochemical Methods
• Globulins: Calculated (Total Protein - Albumin)
• Urea (BUN): Enzymatic (urease) or direct chemical methods
• Creatinine: Jaffe reaction or enzymatic methods
• Uric Acid: Enzymatic (uricase) or chemical methods
• Ammonia: Enzymatic or ion-selective electrode (ISE) methods
• CSF Protein: Biuret or dye-binding methods
• Urine Protein: Turbidimetric or dye-binding methods
• Protein Electrophoresis: Separates proteins by charge/size
• Immunofixation Electrophoresis (IFE): Identifies monoclonal proteins
• Tumor Markers: Immunoassays
• Cardiac Markers: Immunoassays

Special Precautions

Patient preparation and handling the specimens correctly are the groundwork for reliable results

• Patient Preparation: Fasting, medication review, hydration status, posture
• Specimen Collection
  • Correct specimen and tube type
  • Proper order of draw
  • Minimize tourniquet time
  • Clean venipuncture technique
• Specimen Handling and Processing
  • Process specimens promptly
  • Proper centrifugation techniques
  • Appropriate storage conditions (temperature, light exposure)
• Analytical Considerations
  • Reagent Quality: Check expiration dates and storage conditions
  • Calibration: Use appropriate standards and calibrate regularly
  • Quality Control (QC): Run controls regularly, evaluate results
  • Instrumentation: Proper instrument maintenance and function

Troubleshooting

Recognizing what causes errors will ensure the accuracy of measurements

• High or Low Results
  • Pre-Analytical: Patient factors, collection/processing errors
  • Analytical: Reagents, calibration, interferences, instrument malfunction
• Inconsistent Results
  • Review patient preparation
  • Check collection, processing, reagents, calibration
  • Consider interfering substances
  • Repeat the test, use a different method
• Out-of-Control QC
  • Check QC materials, reagents, calibration
  • Repeat QC testing
  • Contact manufacturer if problem persists

Interfering Substances

Endogenous substances or exogenous contaminants can throw off lab results. Being aware of potential interferences can minimize error

• Endogenous: Lipemia, hemolysis, bilirubin, high protein concentrations
• Exogenous: Medications, anticoagulants, contaminants
• Minimizing Interferences
  • Use methods less susceptible to interference
  • Employ techniques to remove or correct for interferences
  • Dilute the sample (if appropriate)
  • Use blanking or background subtraction

Clearances

Clearance tests measure the rate at which a substance is removed from blood by the kidneys, reflecting kidney function

• Creatinine Clearance
  • Collection: 24-hour urine collection, serum sample
  • Principle: Measures the rate at which creatinine is removed from the blood by the kidneys
  • Formula: C = (U x V) / P
  • Estimating GFR: Estimated Glomerular Filtration Rate (eGFR) Equations
• Urea Clearance
  • Urine is collected for one hour
  • Urea concentrations are measured to determine kidney function