How do you make a 1/10 dilution of a serum sample with saline for a Creatine Kinase (CK) assay when the initial result is elevated and out of range?

Making a 1/10 Dilution for Serum CK Assay

To make a 1/10 dilution of a serum sample with saline for a CK assay, you should use 0.05 mL of sample and 0.45 mL of saline diluent.

Rationale for Dilution Calculation

When preparing a 1/10 dilution, the sample represents 1 part out of a total of 10 parts in the final solution. Since the analyzer requires a minimum volume of 0.5 mL, the calculation is:

• Total volume needed: 0.5 mL (minimum required by analyzer)
• Sample volume: 0.5 mL ÷ 10 = 0.05 mL
• Diluent volume: 0.5 mL - 0.05 mL = 0.45 mL

Proper Dilution Technique

1. Obtain a clean test tube or appropriate container
2. Add 0.05 mL (50 μL) of the serum sample using a calibrated pipette
3. Add 0.45 mL (450 μL) of saline diluent using a calibrated pipette
4. Mix thoroughly but gently to ensure homogeneity without causing hemolysis
5. Use this diluted sample for the CK assay

Important Considerations

• Accurate measurement: Use calibrated pipettes to ensure precise volumes of both sample and diluent 1
• Proper mixing: Ensure complete mixing without introducing air bubbles or causing hemolysis
• Dilution factor: When reporting results, multiply the measured value by 10 to account for the dilution
• Sample integrity: Avoid contamination of the sample with saline or heparin prior to making the dilution, as this could affect results 1
• Timing: Process the diluted sample promptly to prevent degradation of the CK enzyme

Common Pitfalls to Avoid

• Using saline-filled needles for initial sample collection, which would pre-dilute the sample 1
• Inadequate mixing, leading to non-uniform distribution of the analyte
• Calculation errors when applying the dilution factor to the final result
• Using inappropriate diluent (normal saline is the appropriate choice for most clinical chemistry assays)
• Failing to document the dilution factor for proper result interpretation

This 1/10 dilution approach is standard practice for handling samples with analyte concentrations that exceed the upper measurement limit of the analyzer, ensuring accurate quantification of high CK levels.