External quality control is performed to ensure the reliability of test results between different laboratories. It is also required by CLIA for laboratory accreditation. External quality control is generally accomplished through proficiency testing (PT). In proficiency testing, simulated patient samples are sent out to laboratories for testing. The CLIA standards for handling proficiency testing specimens are as follows:

• PT samples must be tested with the laboratory's regular patient load.
• PT samples must be tested the same number of times that patients' samples are tested routinely.
• Laboratories participating in PT programs must not engage in interlaboratory comparison of PT sample results.
• Laboratories may not send PT samples to another laboratory for analysis.
• Laboratories must document all steps of processing for PT samples.
• PT is required for only the primary method used for testing of analytes in patients' samples during the period covered by the PT event.

In return for their participation, the laboratory will receive the following information:

• results for each analyte sample
• mean result for each analyte
• standard deviation of results by the comparative method
• number of laboratories using the same method
• standard deviation index (SDI)
• lower and upper limits of acceptability of results

PT results that are between the lower and upper limits of acceptability are considered satisfactory. For chemistry, 80% of samples must test within the acceptable range for the PT to be considered successful.

External quality control serves several purposes, including:

• providing a check on internal quality control
• detecting errors in a lab's methods
• providing a comparison of testing methods, which is useful in selecting new methods