Reasons for Applying Detection Limit Assessment

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

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.

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 result cannot be determined with confidence. 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 test (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.

An upper scheme, as it is known in CCLAS, is really another test that can be performed on a sample to obtain an accurate result at higher concentrations of the analyte, and can be defined by an upper scheme code and upper analyte code.