How to Setup Detection Limits
In analytical chemistry lower detection limit is the lowest quantity of a substance that can be distinguished from the absence of that substance with a confidence interval. It is usually estimated from the mean of the blank and the standard deviation of the blank including some confidence factor. There are a number of different detection limits which are used and they include instrument detection limit (IDL) the method detection limit (MDL), practical quantification limit (PQL) and limit of quantification (LOQ). In LIMS users have an ability to capture some of these limits within the method/parameter set-up. This tutorial will cover set-up of the lower and upper method detection limits in LIMS.
1. Set-up of Upper and Lower Detection Limits in Method Manager
Navigate to the method/parameter for which to set detection limits. If this is a new method, it can be edited, if it's an existing method that is likely in use for multiple different items it will need to be revised. Please follow tutorial "Revising an Existing Method" to revise the method.
- Type in the lower detection limit value into the Lower Detection Limit column (where applicable)
- Type in the upper detection limit value into the Upper Detection Limit column (where applicable)
- Press save within the Parameter pane to save your data
- Press thumbs up button to approve the method for use
2. Revise Items (SampleTypes)
- When a method is revised to add detection limits, LIMS does not automatically revise all the Items (Sampletypes) that use that method to include the most recent version. Revision to items will need to be done manually. If there are over 10 items that require revision, please contact the LIMS administrator to help. To find out what items use the method click on Crystal Reports button in the Method Manager. NOTE: If you have already revised the method, you have to navigate to the previous method version to see which items use it.
- Click magnifying glass next to report "Sample Types Associated with Methods.rpt"
- See a list of all the items that use this method, these items will all need to be manually revised to use the latest method version. See a separate tutorial on how to revise sample types "SampleType Revision"
3. Add Method to New Item (SampleTypes)
- Click on Add New button
- Type in method name
- Click on binoculars to search for method
- Click into the checkbox to select method to add
- Click select to add
If the method you are adding has been revised and approved you will see the method version value based on what revision of the method it was. If the method you are adding is new the method version will be 1. If you forgot to approve your method after revision, you will be adding the old version of the method so be careful with this step and make sure version value is correct. If you forget to approve a new method, you will not be able to find it.
4. Revising Item (SampleType)
This step is better described within the "SampleType Revision" tutorial. Only the critical first step is mentioned below to ensure you choose to apply "approved method/parameters" when revising the item.
- Click on Revision button within the item you are working to revise
- Click to select "Use approved method/parameters"
- Click "OK"
Revised Item (Sempletype) will have approved methods, which means it will apply your most recently revised method with detection limits. You might have to redefine the specification limits, print flags and units for the parameters.
5. Detection Limit in Sample Results
When a result value is typed into LIMS Result column it is compared to predefined detection limits. When the result value is below the lower detection limit, LIMS will automatically update the value to show as "<" and detection limit value.
- Type in value from the analytical instrument
- Click the save button
- LIMS takes the result and compares it to what has been set-up in Method managers. When the parameter has a lower detection limit set-up, LIMS will check if the result value is below the Lower detection limit. If the result is below lower detection limit LIMS will update the value to contain the "<" symbol and insert detection limit value. When parameter has an upper detection limit set-up LIMS will check if the result value is above the upper detection limit. If the result is above the upper detection limit LIMS will update the value to contain the ">" symbol and insert the detection limit value.
In the example above Copper (Cu) parameter has a lower detection limit set at 0.0001 in method manager. When the user entered result of 0.000045 that is less then the lower detection limit so LIMS will automatically update it to <0.00001 when save button is pressed. Result above the lower detection limit will not be replaced.
6. Advantage of Detection Limit Set-Up
6.1. Verification of Numeric Specs
When users enter an actual result from analytical instrument, the result is saved as a numeric and it will be validated against numeric specifications. Even when it's updated by LIMS to show the "<" and detection limit such as <0.0001 it is still saved as a numeric value in the database and is evaluated against numeric specs.
When a user enters the "<" symbol and types in detection limit themselves without relying on detection limits in LIMS, that result is captured as a text value and it is not evaluated against numeric specs.
6.2. Capture and Store Actual Instrument Results
Even when LIMS automatically updates the value of results to "<" detection limit, it will still save original value put in by the users in the database and this can be extracted using crystal reports. When an operator types in "<0.0001" the actual instrument value is not captured in LIMS.
6.3. Avoid Manual Comparison to Detection Limits
When users do not set up detection limits in LIMS they have to always be mindful of entering results for contaminant metals and manually compare the results to detection limits outside of LIMS. This has to be done every time results are being recorded into LIMS. Setting up detection limits take some effort during set-up, but saves a lot of time going forward because you would not have to do the manual comparison for each sample going forward.


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