Accuracy of PCR tests (FOI)
Accuracy of PCR tests (FOI)Produced by the Freedom of Information office
Authored by Government of Jersey and published on 27 April 2021.
Prepared internally, no external costs.
What is the accuracy of the PCR test? And how did you get this accuracy as a percentage, where is the independent data you used to calculate this?
PCR testing in Jersey General Hospital Pathology Laboratory
The Cepheid GX test we use in the hospital laboratory is highly sensitive and specific as it used two markers to look for the virus, the E gene and the S gene.
The method used in the hospital is CE marked which means that it has gone through independent checking of its performance to ensure it performs up to European standard.
The Cepheid Gene expert test we use was evaluated using 45 SARS-CoV-2 positive and 45 SARS-CoV-2 negative NP swab specimens.
The results were used to calculate a Positive Percent Agreement (PPA) and a Negative Percent Agreement (NPA) of this test. The PPA was 97.8% and the NPA was 95.6%
The manufacturers product information also quotes a surrogate marker of accuracy the Lower Limit of Detection (LOD) which is the lowest accurate level of virus detected.
The manufacturer stated LOD for the assay is 250 copies/mL. This laboratory is able to confirm that this is the Lower limit of detection we actually obtain.
PCR testing at the GOJ Covid-19 Laboratory (OpenCell)
OpenCell provide PCR testing for the Island of Jersey for non- in-house hospital patients.
rt-PCR tests are conducted which are a combination of two core processes:
viral RNA Extraction using the Maxwell® HT Viral TNA Kit. This uses magnetic beads to extract the viral RNA qPCR for preparation and analysis
the CE marked RealStar® SARS-CoV-2 kit is being used for the rt-PCR process. The qPCR machine is a qTower from AnalytikJena which performs 40 thermal cycles
The results of 250 Negative and 150 Positive samples have been compared with a UKAS accredited laboratory in an independent cross-laboratory comparison study.
All Evaluation Criteria were met or exceeded, with statistical analysis yielding a Sensitivity of 97%, Specificity of 100%, PPV of 100%, and NPV of 98%.
OpenCell have created a YouTube channel which gives insights into different biotechnologies and applications including rt-PCR testing and our laboratories. Please see the most relevant:
Walk-through CONTAIN Laboratory
An overview of automated RNA extraction
PCR Testing for rapid COVID detection - How to
As with any test accuracy and specificity are dependent on the quality of the specimen taken. An accurate result can only be obtained from a properly taken specimen.