What should be done with specimens with values outside the linear limits (analytical measurement range) of an assay?

Handling Specimens with Values Outside the Linear Limits of an Assay

Specimens with values outside the linear limits (analytical measurement range) of an assay must be diluted to bring the concentration within the reportable range.

Understanding Linear Limits in Laboratory Testing

The linear range or analytical measurement range (AMR) of an assay defines the span of concentrations over which the test provides reliable, accurate results. This range is established during assay validation and represents the concentrations where:

• The relationship between analyte concentration and instrument response is linear
• Precision and accuracy meet predetermined acceptance criteria
• Results can be confidently reported

Why Linear Limits Matter

When specimens contain analyte concentrations above the upper limit of the AMR:

• The detector may become saturated
• The relationship between concentration and signal becomes non-linear
• Results become increasingly inaccurate and unreliable
• Small changes in PCR efficiency can cause significant quantification errors 1

Proper Handling of Out-of-Range Specimens

For Specimens Above the Upper Limit:

1. Dilution is required - The specimen must be diluted with an appropriate diluent to bring the concentration within the validated linear range 2
2. Calculate the final result by multiplying the measured value by the dilution factor
3. Document the dilution factor in the laboratory information system
4. Verify that the diluted specimen result falls within the validated AMR

For Specimens Below the Lower Limit:

• Report as "less than" the lower limit of quantification
• Do not report numerical values below the established lower limit
• Consider using more sensitive methods if clinically necessary

Laboratory Validation Requirements

Laboratories must establish protocols to determine:

• Linear range for each assay
• Analytical measurement range
• Lower limit of quantification
• Procedures for handling values outside performance limits 2

Common Pitfalls to Avoid

• Never report numerical values outside the validated range - This violates good laboratory practice and can lead to misinterpretation of results 1
• Don't recollect specimens unnecessarily - Dilution is the appropriate first step for high-concentration specimens
• Avoid re-centrifuging as a solution - This doesn't address the fundamental concentration issue
• Don't leave specimens for the next shift - This delays reporting and could affect sample stability

Quality Control Considerations

• Dilution procedures should be validated to ensure accuracy
• Regular calibration verification should be performed following any major service to instruments or when using new reagent lots 2
• Document all dilution steps in the laboratory record

Conclusion

The proper handling of specimens with values outside the linear limits is essential for accurate laboratory results. Dilution is the standard approach for specimens exceeding the upper limit of the analytical measurement range, allowing for accurate quantification while maintaining the integrity of the testing process.