Why do we need QC?

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.

Quality of the sample

How the QC or patient sample is obtained, i.e., the pre-analytical component, is arguably the most critical and important step.

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.  

What is a QC sample?

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. The results must fall within 95 - 105% of the known values.

All QC results must be documented, dated and initialled on the relevant daily maintenance & QC worksheets provided.

NOTE: as part of the daily maintenance, QC and QA procedure, ALL changes in lot numbers and expiry dates of consumables- electrodes, reagents, control values, etc, must be documented, dated and initialled on the appropriate worksheets provided.

What does a correct QC result tell you?

Obtaining a correct QC results tells us that:

• the operator knows how to perform the procedure successfully

• the sample has been delivered into the POCT system successfully

• the equipment has been calibrated correctly

• the electronic component (if applicable) is working satisfactorily

• the consumable component, e.g., the cartridge or test strip, is functioning correctly

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 must be competent. They also 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:

• a clot in a blood gas analyser, and subsequent repair

• a reagent replacement

• an electrode replacement

• a change of cartridge or test strip lot number

• a change of test strip with matching code chip 

• the recommended time period between calibrations has been exceeded

After repairs or adjustments are completed, a calibration must be performed and a QC sample MUST be analysed before patient samples are analysed. 

QC Failure and what to do about it

The different types of QC

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:

IQC

Internal QC is where a daily sample, whose values are known, is analysed. Results must fall within "confidence limits", i.e., 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- and inter-lab QC

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 also known as "Sample Comparisons", "Correlations" or "Proficiency Testing".  

The CDHB POCT Coordinator regularly analyses whole blood samples on a variety of analysers capable of measuring the same test. This practice of correlation, checks POCT results against laboratory results, ensuring standardisation is maintained.

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:

• blood gases/electrolytes/metabolites

• glucose meters

• glycated haemoglobins

• haemoglobins

• PR/INR

• urine test strips (Clinitek 50)

External Quality Assurance

EQA is where samples are assayed and results sent to a third party for statistical analysis. The statistical results thus obtained are compared against other participants in the QA program to give an indication of ranking.

Canterbury Health Laboratories participate in an EQA program known as the "AACB-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 the Pacific Rim.

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.