PB2273 LABORATORY PERFORMANCE OF SERUM HOLOTRANSCOBALAMIN ASSAY IN THE UNITED KINGDOM (UK) AS ASSESSED BY THE UK NATIONAL EXTERNAL QUALITY ASSESSMENT SCHEME FOR HAEMATINICS
Authors: Devalia V. , MacKenzie F. 
Published: June 2019

Background:

Although serum B12 assay is the most commonly used routine test for assessing cobalamin status in the body, it has its limitations (Mackenzie F and Devalia V (2018) Laboratory performance of serum B12 assay in the United Kingdom (UK) as assessed by the UK National External Quality Assessment Scheme for haematinics: implications for clinical interpretation. BLOOD, 132, suppl 1, 2230). Holotranscobalamin assay is an alternative test which is increasingly used as a first line test since it is felt to represent the assessment of the functional component of the cobalamin status and possibly a more relevant assessment clinically. However, its technical performance in terms of reliability and suitability in a routine diagnostic laboratory for clinical assessment has not been published. In the United Kingdom, there around 31 laboratories performing the assay using five different platforms. The numerical value obtained of any sample can vary considerably between the different methodologies used. It is not quite clear what would be regarded as the normal or reference range, but it has been previously taken as 40 – 200 pmol/L. External quality assessment of the assays is organised by UK NEQAS for Haematinics by sending three serum samples every 3 months. An ‘all participants’ consensus mean is calculated and used as the target value and the results analysed with respect to inter and intra‐group variation. The percentage bias from the target value is used to assess performance.

Aims:

We present data on one such assessment to demonstrate the performance of the serum holotranscobalamin assay and also how it is interpreted by the laboratory for clinical use.

Methods:

Three serum samples with a HoloTC target value of approximately 16.89 (low/indeterminate), 44,7 (normal) and 72.4 μg/L (high normal) were sent to participating laboratories for analysis. Laboratories were also asked for an interpretation of their result which would be reported to the requesting clinician, namely from low to high.

Results:

Fig 1 shows an individual laboratory's result (indicated by the arrow) in relation to all laboratories using the same technology (shaded histogram) or all methods (open histogram). There is a significant variation with an overall co‐efficient of variation of around 11% within all the three different samples. Fig 2 shows the distribution of results in the different methodologies used and how each laboratory interpreted its result. It demonstrates the bias of results obtained by the different methods. The assays using Beckman Coulter Access/Dxl (SF5 and SF6) gave much lower results than other methods. In the group SF5, three laboratories interpreted the result as ‘normal’ and one as ‘indeterminate’. In the groups Abbot Architect (AB13) and Roche Cobas/Modular (BO5), although having higher results than other groups, there are significant laboratories interpreting the result as ‘low’ and also ‘normal’ or ‘indeterminate’.

Summary/Conclusion:

These data demonstrate that serum HoloTC assay has an overall co‐efficient of variation around 11%. The numerical value obtained of any sample can vary considerably although the clinical interpretation provided by the laboratory does not seem to be markedly discordant. Conclusion: The UK NEQAS Haematinics Programme is unique in providing external quality assessment for laboratories using HoloTC assays for determining body cobalamin status in a style that is also unique across EQA/ PT schemes. As a result, laboratories are made aware of their performance in assessing serum HoloTC level in order to provide appropriate clinical advice.

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