Control Materials in Quality Control Programs

Control Materials (Control Sera)

• Used in quality control (QC) programs
• Defined as a sample tested daily during testing
• Called control sera
• Quality control methods divided into:
  • Internal quality control
  • External quality control

Internal Quality Control

• Performed by the lab itself
• Achieved in two ways:
  • Control sera (pooled sera/specimen)
  • Commercially prepared control sera

Pooled Sera

• Used to check for bias (inaccuracy) in tests
• Preparation process:
  • Collect specimens from the lab with normal results in a screw cap container
  • Exclude specimens that are icteric (jaundiced), cloudy (lipemic), pink (hemolyzed), or with abnormal color (dyes)
  • Perform HB antigen test on all specimens
  • Freeze collected pooled sera daily, up to 1 liter or enough for 4 to 6 months
  • Let the pooled sera dissolve completely at room temperature
  • Transfer to a large bottle and mix thoroughly
  • Send pooled sera for hepatitis antigen test
    • If positive, discard sera
    • If negative, continue preparation
  • Add preservative (100 mg sodium fluoride/dl of serum or 1 ml sodium borate merthiolate/dl of serum)
  • Mix well
  • Centrifuge for 30 minutes to precipitate fibrin or debris
  • Carefully remove supernatant
    • If fats appear at the surface, remove them using cotton or glass wool to clear the serum
  • Divide the supernatant into small (about 1 ml) containers, label, and freeze at -20°C or less

Commercially Prepared Control Sera

• Available as freeze-dried (lyophilized) or liquid synthetically manufactured sera
• Freeze-dried control sera:
  • Preserved at 2-8°C
  • Read instructions before use
  • Open slowly to avoid loss of substances
  • Use good quality glassware and special diluents
  • Mix well; leave at room temperature for 5-10 minutes; distribute into small sterile containers (0.5-1ml)
  • Do not use manual pipettes to avoid contamination
  • Label, freeze, and store at -20°C until needed

Comparison of Pooled and Commercially Prepared Sera

Control (pooled + commercially)

• Detect errors in reagents and standard test dilutions (STDs), but not individual pipetting or calculation errors

Quality Assurance (QA) vs. Quality Control (QC)

External Quality Control

• A quality checking system using an external organization
• Not a substitute for internal quality control because it can only assess past performance

What is an Error?

• Failure of a planned action to achieve its desired goal without external factors causing the failure

Analytical Errors

• Errors of Bias (inaccuracy, systematic, regular):
  • Unsatisfactory reagents or standards (preparation, storage, impure, expired)
  • Incorrect calibration
  • Incorrect wavelength
  • Poorly written procedure
• Errors of Scatter (imprecision, random, irregular):
  • Faulty technique (pipetting, mixing, incubation)
  • Dirty tubes, pipettes, or glassware
  • Heavy or low workload
  • Fluctuating electricity and temperature
  • Finger spots on cuvettes and air bubbles
  • Incomplete removal of interfering substances (e.g., red blood cells (RBCs))