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| Point of Care Testing |
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| Point of Care Testing |
POCT Quality Control (QC)
| Quality Control applies to all laboratory testing, including POCT. | |||||||||||||||||
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We need to regularly analyse a QC sample so that we can have confidence in the patient's results. With correct QC results- and only if they are correct- we can assume that the patient results will also be correct.
How the patient sample is obtained is arguably the most critical and important step in laboratory and POC testing. This link gives a detailed discussion of pre-analytical components of laboratory testing.
A poor quality sample will give poor quality results.
Since most POCT samples are capillary blood samples, a good capillary blood collecting technique is crucial.
Regardless of the type of sample required- whole blood, urine, stool- a high quality, non-contaminated, properly collected sample is mandatory.
Only if the three crucial components...
...are each of a high standard, can we be assured that the patients' results will be accurate, precise, reliable and comparable to the laboratory.
A Quality Control sample is usually made of a liquid, either aqueous or blood-based in nature. Its values are already known. It is analysed in exactly the same way as a patient sample.
After analysis, each result of the QC sample is compared to its known value. If the results all fall within 95 -105% of the known values, it may be assumed that the analyser or process is functioning correctly and that the patient results will be correct to a similar degree.
All QC results must be documented, dated and initialled on the relevant worksheets provided.
What does a correct QC result tell you? Obtaining a correct QC results tells us that:
For a process where no analyser is involved, e.g., visual urine test strip reading, analysing a control regularly ensures that the strips are working correctly (they can be spoiled by contamination from humidity. Keep the lid on tight!). A QC analysis also checks the relative colour perception and the ability of the operator. For more information regarding urine test strip QC, please contact the POCT Coordinator.
When should a QC sample be analysed? Routinely, a QC sample should be analysed and the result found to be acceptable, at the start of each day- after any daily maintenance, required calibrations and so forth have taken place, but before any patient samples are analysed.
If the analyser or process is used less frequently, say once or twice a week only, then a QC sample needs to be analysed only on the days that patient samples are analysed. Once again, when the QC sample is analysed, it is always after daily maintenance and calibrations, but before the patient samples are presented.
What does all this have to do with POCT? POCT analysers must be operated with the same degree of care that would be expected of laboratory analysers operated by laboratory professionals. All operators of POCT equipment- nurses, technicians and doctors have a responsibility- a Duty of Care- to ensure that the POCT results they generate are accurate and precise. This means, in part, that QC samples must be tested regularly. How can it be assumed that the patient results are correct unless the QC results are acceptable?
Circumstances where more frequent QC is required Sometimes, a change in the condition of the analyser or POCT process can occur. Some examples are:
After repairs or adjustments are completed, a calibration must be performed and a QC sample MUST be analysed. No patient samples must be analysed until this process is successfully completed with QC result(s) within the expected ranges.
NOTE: ALL changes in lot numbers, electrodes, reagents, control values, etc, must be documented on the appropriate charts provided. Lot or batch numbers, expiry dates, date and time of change and operator ID must be listed.
QC Failure and what to do about it
So far, the discussion has outlined what is referred to as "Internal QC" (IQC), i.e., routine daily QC requirements.
There are other types of QC depending on conditions and circumstances. A formal definition of IQC and the other types follows:
Internal QC is where a daily sample, whose values are known, is analysed. Results must fall within "confidence limits", ie, between 95% and 105% of the Mean Value. This can also be expressed as plus or minus two standard deviations (± 2SD) of the Mean for each value in question. These results are monitored on a daily basis (see diagram of Levy-Jennings graph).
The results are examined for shifts in value; trends in value (3 or more points in sequence moving away from the mean); and precision limits.
Intra-lab QC is defined as samples shared and analysed on instruments within the same laboratory, e.g., a sample compared between two blood gas analysers. Inter-lab QC is defined as samples shared and analysed on instruments in different areas where data can be compared and correlated. This process is sometimes referred to as "Proficiency Testing". It might concern two different laboratories with different instruments, attempting to resolve a problem with an analysis. An example with a POCT flavour might be for a whole blood sample to be analysed for haemoglobin in the laboratory. Then the same sample is analysed on haemoglobin meters in clinical areas in the same hospital (or somewhere else). Results are then compared. There should be very little difference between any sets of results. This is also referred to as Proficiency Testing. Canterbury Health Laboratories participate in an external quality control program known as the "RCPA Survey" (Royal College of Pathologists of Australia). The RCPA provides statistical analyses of QC results that we send to them on a regular basis, using median values derived from several hundred participating laboratories throughout Oceania. The RCPA offers many different external QC modules for various areas of testing: serum, urine, endocrine, drugs of abuse, etc. As well as the former, Biochemistry subscribes to the blood gas module; Biochemistry and POCT blood gas analysers are tested once per month using aqueous QC samples containing albumin to simulate a whole blood matrix. pH, pCO2, pO2, sodium, potassium, chloride, ionised calcium, glucose, lactate and the various types of haemoglobins and dys-haemoglobins can be reported.
The POCT Coordinator regularly analyses random patient samples on a variety of analysers capable of measuring the same test. This practice of correlation, checks on the standardisation of all results, regardless of differing methodologies.
POCT analysers or processes giving results outside of acceptable limits must be examined for the cause of the error. The error must be resolved before further patient tests are reported. All errors and their resolution are documented for further reference.
All completed correlations are stored in the POCT Manual in the POCT Coordinator's Office. Patient correlations are analysed fortnightly or monthly. The range of processes includes:
Consumable and reagent quality Reagents, cartridges and other consumables must not be used after their expiry date and be discarded.
Lot numbers of reagents, cartridges and other consumables will change occasionally. A calibration must occur after any changes are made to the system, even if lots numbers are identical.
A QC analysis must be made after any calibration. The QC must be correct before any patient samples are analysed.
All consumable changes and lot numbers must be documented on the appropriate Consumables Worksheet for the test in question (see the Appendix of the Operator's Manual for documentation and mastercopies).
Maintenance and troubleshooting records All maintenance and troubleshooting logs are retained and kept for an extended period of time. These logs list reagents and consumables used, QC results, problems solved and parts replaced. Each day's maintenance is signed off by the laboratory personnel or accredited representative involved.
More information is available on the Quality Assurance page.
For any additional advice, assistance, lectures, troubleshooting issues or meter replacements, please contact the POCT Coordinator. |
