Proficiency testing (PT) is the most common method used by clinical laboratories for ascertaining the accuracy of their test results. In this, the Proficiency Testing Provider (PTP) evaluates performance of participant laboratories against pre-established criteria by means of inter-laboratory comparisons (ILC). It helps in identification of problems/errors that could otherwise result in reporting of wrong reports. PT also helps in establishing comparability of test or measurement methods, gain confidence on reports and for identification of inter-laboratory differences based on method or equipment.
Participation in PT is an essential requirement for a laboratory going for accreditation. As per NABL 163, a laboratory should successfully participate in at least one PT program prior to gaining accreditation in each discipline applied.
Interlaboratory comparisons done between two or few labs has a number of limitations like homogeneity and stability of PT items is not established, handling, transportation & storage may not be standardized i.e. sample integrity may be compromised, possibility of collusion and falsification, tendency to hide poor performance, etc. Hence, it is preferred to participate in a PT program, wherever available for facilities that perform moderate- or high-complexity testing Tests classified as "waived" are generally exempt PT requirements.
CLIA in USA for most analytes, stipulate that PT programs must include at least three events annually, each of which must include five samples. With the exception of transfusion medicine testing where the only passing score is 100%, a “passing” PT score (satisfactory performance) requires that 80% (4/5) of a participating laboratory’s test results fall within a specified range of analytical precision, defined either in relation to a “known” value established with a reference method or in relation to the values measured by a peer group of laboratories or a group of reference laboratories.
PT programs, provide samples to laboratories which are examined and values reported back to PT provider and results of all participant labs evaluated and individual reports issued to each laboratory, are known to show improvement in laboratory performance due to elimination of chronic poor performers from the pool of laboratories participating in PT for a given analyte, or correction of chronic problems by laboratories that remained in the pool. Chronically poor PT performance may motivate a laboratory manager to adopt a new instrument or method that is intrinsically more accurate or reliable. Improved education and performance may be the result of hiring more qualified personnel but may also result from an effective use of PT results and performance to educate laboratory personnel.
Types of PT schemes:
PT programs can be divided into various type basis the following criteria:
- Type of results - qualitative or quantitative
- Distribution of PT items - simultaneous (PT items are distributed for concurrent testing), sequential (PT items are distributed sequentially or are returned to the PT provider at intervals)
- Frequency - continuous (PT items are provided at regular intervals) or single occasion
- Interpretation based program
- Sampling type
PT programs offered by various providers may have variations in following areas:
- Discipline: PT is available for almost all the disciplines which include Clinical Biochemistry, Haematology, Microbiology & Serology, Histopathology & Cytopathology, Clinical Pathology & Molecular Biology. For many tests it may not be available, due to issues of instability, matrix limitations, etc.
- Rounds per cycle: varies from monthly/ quarterly/ biannually/ annually. Requirement is to have at least one PT round per year for any given parameter
- Number of PT items: Most of the providers give one sample per round, only few give multiple samples per round that helps a laboratory to differentiate a true outlier from a random defect.
PT items
These are the samples sent by the PT providers to labs for evaluation. Matrix provided includes lyophilized material, serum, whole blood, EDTA blood, plasma, urine, stool, tissue samples, images, whole slide images etc. Aim is to prepare a homogenous and a stable material which mimics the actual patient sample. However, this is not always possible hence the need for evaluation according to peer/ method/ mode becomes important for many analytes.
PT Evaluation
Participant results are evaluated basis the ‘Assigned value’ established for each parameter. Following methods are used for fixing the assigned value and the associated uncertainty:
- Quantitative scheme:
- Consensus value of participants - this is the most common method used by providers for Medical Testing Laboratory.
- Formulation – used when different materials are mixed in a fixed proportion or added to a base material.
- Certified Reference Material – used when a certified reference material is the PT item.
- Results from a single laboratory
- Consensus value of expert laboratories
The results are calculated basis the closeness of the participant results to the assigned value. The formula is: (Lab result-Assigned value)/ Standard deviation of the proficiency round {SDPA}
Note: Uncertainty due to inhomogeneous PT item, effect of transport conditions and instability of the material can be factored into SDPA.
- Qualitative scheme: These schemes behave differently than the quantitative schemes in terms of result input and fixing an assigned value
Result input:
- Categorical (nominal) scale - here the property has no magnitude; e.g. Name of the bacterium
- Ordinal scale - this can be dichotomous; such as: Positive/ negative or reactive/ non-reactive; or it can be quantified but the values do not have an arithmetic relationship. For e.g. +1/+2
Ascertaining Assigned value:
- Expert opinion – For results relying on microscopy, for diagnosis, AV can decided by a panel of experts or an individual expert. PT provider has to assure consistency of diagnosis.
- AV for the reference material
- From knowledge of origin or preparation of the PT item
- If the results are categorical or ordinal more than one sample per round or replicate testing by participants should be considered and the AV can be assigned using mode or the median of the participant results or results given by 80% or more of the participant’s.
PT programs usually provide samples to laboratories, which are examined and values reported back to PT provider and results of all participant labs evaluated and individual reports issued to each laboratory. As per ISO/IEC 13528: 2015 the participants should review their z scores on the following lines:
- Z score < 2.0 indicates “Acceptable” performance and indicates no action signal.
- Z score ≥2.0 but < 3.0 indicates “Warning” & needs to be reviewed.
- Z score ≥ 3.0 indicates “Unacceptable” performance & generates an action signal.
Qualitative: Results can be evaluated as Acceptable/ Unacceptable
In conclusion, PT programs are an essential component of Quality Assurance Plan to implement continuous improvement and achieve better outcomes which is helpful in better healthcare delivery for patients and users of laboratory services.
