Assessing Results against Detection Limits

Overview

The measurement of an analyte by an analytical scheme generally has a range of analysis at which it is confident that a result is accurate. This range is delimited by the lower and upper detection limit for analytes within a scheme version of the scheme, and is usually based upon method or instrument limitations. Internal detection limits are present to ensure that the test result obtained for the scheme are within the acceptable limits of detecting that result.

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

Limits of analysis are used to define the range over which a scheme, or more specifically, 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.

Detection limit assessment and using upper schemes is a component of quality assessment applied to unknown and duplicate samples, that is, samples with a Primary Analytical Type of Unknown or Duplicate.

Any scheme version analyte that has a Data Type of Numeric can be bound by:

A lower detection limit of analysis—The scheme version analyte's Internal Lower DL is the value below which any amount of substance that can be distinguished from the absence of that substance, within a stated confidence, and therefore deemed to be not detectable by that test.
An upper detection limit of analysis—The scheme version analyte's Internal Upper DL is the value above which any result is deemed to be not accurate by the test.

A sample scheme analyte's Internal Lower DL and Internal Upper DL is inherited from the scheme version analyte's Internal Lower DL and Internal Upper DL, respectively, and may be updated on individual sample scheme analytes.

When a result is received or cleared for a sample scheme analyte, detection limit assessment occurs. From this, the sample scheme analyte's Detection Limit Status is returned as either Not Tested, Not Required, Passed, Lower Failure or Upper Failure.

If the results are too low, then typically it means the scheme is not suitable for such a small number. This might be a trigger to perform another method that is more suitable for a lower concentration. The sample scheme analyte's Internal Lower DL is separate to the Report Lower DL—when these low results are reported, they are rounded according to the reportable limits, such that, if a result is too low then it would typically be reported with a < prefix in front of the Report Lower DL, as defined by the rounding table used.

If the results are too high, then typically it means the scheme is not suitable for such a large number. This might be a trigger to perform another method that is more suitable for a higher concentration. The sample scheme analyte's Internal Upper DL is separate to the Report Upper DL—when these high results are reported, they are rounded according to the reportable limits, such that, if a result is too high then it would typically be reported with a > prefix in front of the Report Upper DL, as defined by the rounding table used.

Process

Configuring Detection Limit Assessment

Scheme version analytes are configured for detection limit assessment using the CCSCHM—Scheme application, which is also accessed using the Analysis Setup » Schemes menu option.

Create the analytical scheme. Refer to Maintaining Schemes.
Create an analyte to hold an analytical result. Refer to Maintaining Scheme Version Analytes.
A scheme version analyte is configured to undergo detection limit and QC assessment by selecting the scheme version analyte's QC Active flag. If this flag is cleared, then a detection limit assessment of a result is determined as Not Required.

Configure the analyte to be assessed for QC. Refer to Maintaining Scheme Version Analytes

.A scheme version analyte is configured to have an Internal Lower DL (detection limit) and/or an Internal Upper DL.

The Internal Lower DL Critical and Internal Upper DL Critical flags on the scheme version analyte allow for one or both limits to be critical for validation.

When a sample scheme analyte is created for the scheme version analyte, the Internal Lower DL, Internal Upper DL, Internal Lower DL Critical and Internal Upper DL Critical flags are inherited from the scheme version analyte. It is these flags and values on the sample scheme analyte that are used for detection limit assessment.

Then, after the sample scheme analyte receives a result:

Data Type	Result	Internal Lower DL Critical?	Internal Upper DL Critical?	Detection Limit Status	Workflow Status
Not Numeric				Not Required	Analysed
Numeric	<Internal Lower DL	Yes		Lower Failure	Check
Numeric	<Internal Lower DL	No		Lower Failure	Analysed
Numeric	>Internal Upper DL		Yes	Upper Failure	Check
Numeric	>Internal Upper DL		No	Upper Failure	Analysed
Numeric	>=Internal Lower DL where it is defined AND <=Internal Upper DL where it is defined			Passed	Analysed

Detection limit assessment occurs irrespective of the setting of the scheme version analyte's Workflow Active flag (as this setting is to do with rolling the workflow status up to sample scheme level).

Configure an analyte to have a Non-critical Lower Detection Limit, Critical Lower Detection Limit, Non-critical Upper Detection Limit and Critical Upper Detection Limit by Maintaining Scheme Version Analytes.
Where a breached critical limit occurs in which the Workflow Status is left as Check, then the situation is dealt with either by redetermining the result to one that is within critical detection limits, ignoring the critical failure, or setting an upper sample scheme analyte, where one is registered on the sample, to test the sample scheme analyte.
An upper scheme for a scheme version analyte is configured by entering the code of a valid scheme and analyte in the Upper Scheme Code and Upper Analyte Code fields, respectively. These are free-text fields, so care must be taken to enter these codes. Of course, to indicate that there is an upper limit to a scheme, over which the upper scheme is required, the upper detection limit of the analyte within the lower scheme must be entered in the Upper Detection Limit field, and whether the upper detection limit critically affects the workflow status is defined by checking the Upper Detection Limit Critical.
Publish the scheme version. Refer to Configuring Versions of a Scheme..
Where the scheme has org-scope:
1. Enable the org-scope scheme for use in the laboratory. Refer to Using Organisational Schemes vs Laboratory Schemes.
2. To specify the use of org-scope or lab-scope properties on the scheme version during accuracy assessment, Set the WORKBOOK_OVERRIDE_SCHEME_VERSION_LIST Preference.
3. To specify the use of org-scope or lab-scope properties on the scheme version analyte during accuracy assessment, Set the WORKBOOK_OVERRIDE_SCHEME_VERSION_ANALYTE_LIST Preference.

Configuring Samples for Detection Limit Assessment

A sample scheme analyte inherits the Internal Lower DL (Detection Limit) and Internal Upper DL from the scheme version analyte, but they may be updated. The QC Active, Internal Lower DL Critical and Internal Upper DL Critical flags, however, are only maintained on the scheme version analyte.

The following conditions must be met for accuracy assessment to occur, to set the sample scheme analyte's Standard 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 is Duplicate and the Secondary Analytical Type is empty.
The scheme version analyte's Is QC Active is selected.

When a sample is created and tests are added to it, the sample scheme analyte's Internal Lower DL and Internal Upper DL default to those on the scheme version analyte, and can be updated, if required.

The sample scheme analyte's Internal Lower DL Critical and Internal Upper DL Critical flags, similarly, default to those on the scheme version analyte, and can be updated, if required.

Where a template job is used to register a job, or a template sample is used to register a sample, then the sample scheme analyte's Internal Lower DL, Internal Lower DL Critical and Internal Upper DL, Internal Upper DL Critical flags default to those in the template.

A scheme version analyte can be assigned a Lower Template Sample Code and Upper Template Sample Code, such that, if the lower or upper detection limit, respectively, is breached on a sample scheme analyte related to the default scheme version analyte, then the user can trigger the registration of the tests from the particular template sample onto the sample so that check analysis can be performed.

Alternately, a scheme version analyte can be assigned an Upper Scheme and Upper Analyte. The upper scheme and analyte is generally registered for the sample but assigned a Workflow Status of Not Analysed or No Result as there may ultimately be no need to analyse the scheme by this method where the lower scheme's range is suitable based on the result. However, where the upper detection limit is breached on a sample scheme analyte related to the default scheme version analyte and the AUTO_ACTIVATE_UPPER_SCHEME application preference is set, then:

The linked upper scheme analyte is activated, which means that if the upper sample scheme's Workflow Status is Not Analysed or No Result, then it is set to Not Started to indicate that there are tests outstanding for it.
The linked upper analyte is activated, which means that if the upper sample scheme analyte's Workflow Status is Not Analysed or No Result, then it is set to Repeat (rather than to Not Started) so that the sample scheme analyte can be located easily since these results need to be determined more quickly to get the report to the client.
The lower sample scheme analyte's Workflow Status is updated from Check to Analysed to indicate that is completely analysed and that there is nothing further to be done for it.

It is a prerequisite that the upper scheme and upper analyte be registered on the sample and that the upper sample scheme analyte's Workflow Status be either Not Analysed or No Result. If this is the case, then the upper sample scheme analyte's Workflow Status is reset to Not Started and the lower sample scheme analyte's Workflow Status is changed from Check to Analysed.

Whether an upper scheme is automatically activated whenever a sample scheme analyte's Workflow Status is Check is set by preference.

Set the AUTO_ACTIVATE_UPPER_SCHEME Preference

Refer to Conditions for Auto-activation of Upper Scheme Analytes.

The limitation to using an Upper Scheme and Upper Analyte instead of a Lower Template Sample Code and Upper Template Sample Code is that only one scheme analyte is provided for the check analysis.

Refer to Maintaining Sample Tests.

Where a sample scheme analyte is created or its result is cleared, then the sample scheme analyte's Detection Limit Status is set to Not Tested.

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

After a result is entered for a sample scheme analyte in the CCWKOP—Open Workbook application, and the system has performed detection limit assessment to set the sample scheme analyte's Detection Limit Status and contribute to set the sample scheme analyte's Workflow Status, the outcome of that assessment can be viewed in the lower panel. The outcomes of detection limit assessments can also be viewed in the CCVALN—Job and Sample Validation application.