Quality Control Frequency

How often is “right” for Quality Control?

It is widely accepted that laboratories should perform QC at least every day of patient testing. However, is this the right QC frequency for every assay and for every laboratory? Is running QC once per day really sufficient? What is the “right” frequency for running QC samples in your laboratory?

ISO 15189 regulations don’t state a recommended QC frequency but they do recommend that:

“Quality Control materials shall be periodically examined with a frequency that is based on the stability of the procedure and the risk of harm to the patient from an erroneous result.”

ISO 15189 understands that differing tests and situations will require differing QC frequencies. So how do you use this advice to work out the correct QC frequency for the assays in your lab?

There are various factors that you need to consider when deciding an appropriate QC frequency. A good place to start is by asking the right questions:

  •  Which assays are more stable compared to others?
  •  Which tests are higher risk and have a higher impact if results are erroneous?
  •  How many patient samples are you running in between QC evaluations?
  •  What is the time between QC evaluations?

If you ask the “right” questions, you’ll get the “right” answers.

Which assays are more stable compared to others?

Some assays naturally perform better than others, giving consistently better results. On the other hand, some assays perform inconsistently, having a higher rate of error and much lower stability. It’s important that laboratories can recognize which assays are more stable and consistent in comparison to others and ensure that they are running QC at an appropriate frequency.

By utilizing a PT scheme and/or peer group reporting programme, method validation and peer performance comparison can be monitored, helping laboratories to assess their precision over time and easily identify which tests generally perform better or worse. Do this to determine which assays are more unstable and run QC more frequently for those assays.

Which tests are higher risk and have a higher impact for an erroneous result?

It’s important that you run QC more frequently for higher risk tests. With higher risk tests there is a greater risk of harm to the patient, therefore it’s of utmost importance that the results released are both accurate and reliable.

Any tests that have the following characteristics should be considered high risk and QC should be run more frequently in these instances.

  • A test where there could be a detrimental consequence, should the wrong test results be released
  • A test that supports the clinician’s decision in isolation
  • A test that is acted upon immediately
  • A test that is performed on a specimen that is difficult/painful to collect

How many patient samples are you running in between QC evaluations?

Let’s consider a scenario: “Lab A” and “Lab B”, both of which perform QC every morning. That’s sufficient, right?

Lab A tests 10 patient samples a day, whereas Lab B tests 1000 patient samples a day. Is performing QC once per day still sufficient for each lab? Say an error occurred in the test system after 50% of the samples had been tested. Neither labs would recognize any QC failure until the next day, meaning erroneous patient results may have been released. In both cases, they will have to re-evaluate the patient samples from the last successful quality control event as recommended by ISO 15189. Potentially Lab B will have to repeat 1000 patient samples, meaning a significant wastage of both time and resources.

Ideally patient samples should be run in batches, perhaps every 50 or 100 patient samples, starting and ending with a QC evaluation. This will ultimately save time and money, and most importantly will reduce the risk of harm to the patient.

What is the time between QC evaluations?

Let’s consider another scenario. Both Lab A and Lab B now decide to change their QC strategy to run QC every 100 patient samples. Great news, right? This perhaps is a good move for Lab B as QC will be run more frequently, reducing risk for their patients. However, it’s not good for Lab A.

Let’s say that an error occurs after 50 patient samples have been run. For Lab B, they will detect the problem straight away on day 1 and will be able to investigate the problem preventing the release of erroneous patient results. For Lab A, the error will have occurred on day 5 of their patient testing, but the problem won’t be recognized until day 10! This could spell disaster for any laboratory, with the release of potentially erroneous results causing misdiagnosis, incurring cost and resulting in a negative impact on patient care.

Therefore, it’s vital to consider the time between QC evaluations in correlation to the number of patient samples being tested. Keep the time between QC evaluations shorter than the time needed to undertake any corrective action in the case of an erroneous result. This is a good rule of thumb to ensure you select an appropriate QC frequency.

Unfortunately there is no straightforward answer to how frequently you should run QC. However, if you ask the right questions, you’ll reach the right answer. Make sure you are running QC more frequently for high risk and unstable tests; ensure you start and end patient testing with a QC evaluation; and make the time between QC evaluations shorter than the time needed to take corrective action in the case of an erroneous result.

Finally QC samples should also be tested before and after any event that has the potential to adversely affect the testing process e.g. change of reagent batch, instrument maintenance and calibration. Testing prior to the event provides confidence that patient results since the last successful QC check are reliable. Testing QC samples immediately after the event ensures the test system is in control prior to running more patient samples.

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