Assessing Results against Scheme or Surrogate Limits

Overview

Assessing results against scheme or surrogate limits is a component of quality assessment applied to unknown, duplicate and replicate QC samples.

Limits of analysis are used to define the range over which the analytes within a scheme can be measured with confidence. These limits are determined by a number of factors, primary among these being the working range of the calibration standards, instrument detection limits, detector saturation as well as sample effects such as matrix and internal interferences.

In CCLAS 6, the assessment of a result against a scheme limit specification is known as scheme limit assessment, and sets the limit status for the analysis.

If a full-blown scheme limit on the specification is too complex, and simple surrogate limits suffice, then instead of configuring a scheme limit specification on the scheme, surrogate limits can be configured on those scheme version analytes that required them. If the scheme is linked to a limit specification, CCLAS performs a scheme limit assessment to set the limit status for the analysis. Otherwise if the scheme has not been linked to a limit specification, or the specification does not contain limits for the sample scheme analyte, and the scheme version analyte contains a surrogate target and limits, then CCLAS performs a surrogate limit assessment to set the limit status for the analysis.

A surrogate limit for target and range can be set on a scheme version analyte. These can be considered substitute limits, and are used for an analyte where a limits specification does not exist, or is not set for a scheme version analyte. They are often based on method or instrument limitations.

In CCLAS 6, the assessment of a result against surrogate limits is known as surrogate limit assessment, and sets the limit status for the analysis.

Process

Creating the Scheme, Version and Analytes

Set the SCHEME_AUTO_CREATE_LIMITS_SPEC application preference so that when a scheme is created, it links to or creates a matching scheme limit specification.

Schemes are configured using the CCSCHM—Scheme application.

For a sample scheme analyte's result to be assessed for scheme or surrogate limits, the following conditions must hold:

The scheme version analyte must have a Data Type of Numeric, Numeric Only or Text.
The scheme version analyte must have a Unit Code.

Important: This also applies to text analytes, in which case, if there is no unit, create a unit called NO_UNIT and use that, for example.
The scheme version analyte's QC Active must be selected.
For the outcome of the assessment to be written to QC history, the scheme version analyte's Saved to QC History must be selected.

If a scheme's Limit Specification is not defined or has no defined limits for an analyte plus unit, then surrogate limits on the scheme version analyte are used for surrogate assessment.

Adding Limits for the Scheme Limit Specification

Update the scheme limit specification to add the specification limits for the scheme version analyte units. Specifications are configured using the CCSPFN—Specification application.

A specification can contain multiple entries for scheme, version, analyte and unit.

For a sample scheme analyte's result to be assessed for scheme limits, the specification must contain an entry for the related Scheme Code, Scheme Version, Analyte Code and analysis Unit Code.

Important: This also applies to text analytes, in which case, if there is no unit, create a unit called NO_UNIT and use that, for example.

Scheme limit specifications are always treated as critical for validation, regardless of the Critical For Validation setting on the specification scheme version analyte, as the outcome of the assessment always contributes to the Composite QC Status. The Critical For Validation flag is only applicable to product specifications.

Numeric and Numeric Only Analytes

Where the scheme version analyte has an Analyte Data Type of Numeric or Numeric Only, scheme limit assessment uses the specification scheme version analyte's internal absolute limits, where the warning and failure ranges are absolute.

There is the option to enter these limits as relative values, where a percentage around the target value for warning and failure limits is designated, however, the system determines the equivalent absolute values and uses them for the accuracy assessment.

A specification scheme version analyte unit's numeric limits are bound by any or all of the following:

< or = Internal Minimum Value—The lowest acceptable amount of substance for the specification to be satisfied, below which the material would fail the standard lot's specification.
< or = Internal Minimum Warning Value—The lowest acceptable amount of substance for the specification to be satisfied without warnings, below which the material would satisfy the standard lot's specification but with warnings.
Internal Target Value—The target value for the acceptable amount of substance in the material.
> or = Internal Maximum Warning Value—The highest acceptable amount of substance for the standard lot's specification to be satisfied without warnings, above which the material would satisfy the specification but with warnings.
> or = Internal Maximum Value—The highest acceptable amount of substance for the standard lot's specification to be satisfied, above which the material would fail the specification.

Text Analytes

Where the scheme version analyte has an Analyte Data Type of Text, the specification can be configured to assess strings with or without case-sensitivity.

Note: Leading and trailing spaces are not removed upon cell exit but are removed upon submit. Therefore, if a list of expected results in an applied specification contains "DEF" and not "DEF " for a text analyte, and "DEF " is entered into the cell, then upon cell exit, the cell is flagged red to show a failed specification assessment. However, upon submit, leading and trailing spaces are removed, so "DEF" is persisted, such that, upon workbook reload, if the cell undergoes Force Reassessment, then the cell shows a passed specification assessment.

Setting a Scheme Limit Specification on a Sample Scheme

A sample scheme's Scheme Limits Specification Code is inherited from the scheme's Limit Specification Code, and may be updated on individual sample schemes. Surrogate limits are only maintained on the scheme version analyte.

If a template job is used to register a job, or a template sample is used to register a sample, then the sample scheme's Limit Specification Code defaults from the template.

The following conditions must be met for scheme limit specification assessment to occur, to set the sample scheme analyte's Scheme Limit Status when a result is entered:

The sample is either a/an:
- Client sample—The sample's Primary Analytical Type is Unknown and the Secondary Analytical Type is empty.
- Hidden blank QC Sample—The sample's Primary Analytical Type is Unknown and the Secondary Analytical Type is Blank.
- Hidden standard QC Sample—The sample's Primary Analytical Type is Unknown and the Secondary Analytical Type is Standard.
- Duplicate QC sample—The sample's Primary Analytical Type or Secondary Analytical Type is Duplicate.
- Replicate QC sample—The sample's Primary Analytical Type or Secondary Analytical Type is Replicate.

When does Scheme Limit Assessment occur, and what does it set?

After a result is entered or cleared for a sample scheme analyte in the CCWKOP—Open Workbook application, scheme limit assessment (via a scheme limit specification or surrogate limits) occurs after detection limit assessment.

If a sample scheme has a Limit Specification Code defined and there is a specification limit for the matching sample scheme's scheme, scheme version, analyte and analysis unit, then scheme limit assessment occurs instead of surrogate limit assessment.

If a sample scheme does not have a Limit Specification Code defined, or it is defined but there is no specification limit for the matching sample scheme's scheme, scheme version, analyte and analysis unit, then surrogate limit assessment occurs instead of scheme limit assessment.

Refer to Entering Results into Workbook Cells and Forcing the Reassessment of Results in a Workbook Session.

When a sample scheme analyte is created or its result is cleared, then the sample scheme analyte's Scheme Limit Status is set to Not Tested.
Where conditions do not hold for scheme limit assessment, then the sample scheme analyte's Scheme Limit Status is set to Not Required.
Otherwise, the outcome of a scheme limit assessment sets the sample scheme analyte's Scheme limit Status to either Passed, Lower Warning, Upper Warning, Lower Failure or Upper Failure, which contributes to set the sample scheme analyte's Composite QC Status.

The outcome of a scheme limit assessment can be viewed in the lower panel. The outcomes of QC assessments can also be viewed in the CCVALN—Job and Sample Validation application.

The sample scheme analyte's result cannot be validated if there is any failure on the Composite QC Status.

Refer to: