Abstract
Of all the tests undertaken by Clinical Biochemistry and Clinical Immunology laboratories, few can be less standardized than those for cryoglobulinaemia. Sangur et al. 1 have tried to address this inconsistency by means of a position document in this issue of the Annals of Clinical Biochemistry.
It is clear that it has been difficult to agree even a definition of what constitutes a cryoglobulin. Most would agree that immunoglobulins that precipitate out of serum on cooling and re-dissolve on warming to 37°C are cryoglobulins. However, not all cryoglobulins precipitate; some form gels, some crystals and, rarely, some separate almost as oils and appear as two liquid phases. Only those that separate into solid and liquid phases can be properly processed, identified and quantified as suggested, but that does not mean the others, which require care to detect, are unimportant.
As highlighted by Sangur and colleagues, it was worryingly pointed out from a UK National External Quality Assessment Scheme survey that only approximately one-third of all laboratories collect samples in an appropriate manner. That is, ensuring that the sample remains at or above 37°C until it has been separated. It was also noted that there was a huge variability in the time the precipitate was allowed to form at 4°C: from 12 h to 9 days. The type III (polyclonal mixed) cryoglobulins, in my experience, often take three days for reasonable, even if not maximal, precipitation. After 12 h only high concentration cryoglobulins, usually type I (monoclonal) and with a high thermal profile, will precipitate maximally. The pragmatic advice of Sangur and colleagues is to allow precipitation to occur for no less than three days at 4°C. This would at least ensure detection of the type I and type II cryoglobulins with some expectation of success. Some type III may take longer and this is the basis for the suggestion that seven days is preferred.
From the laboratory perspective, perhaps one of the most useful reminders in this review is the ‘indicators for investigation’ box. We should be on the lookout for anomalies without other explanation and alerting our clinical colleagues accordingly. Very low serum complement component C4 (≤0.1 g/L) is often reported by laboratories without comment. There are, of course, reasons other than cryoglobulin for a low C4 (genetic deficiency, immune complex disease) but in our experience approximately 80% of patients with cryoglobulinaemia present with a serum C4 of ≤0.1 g/L. It is our standard advice to clinicians to consider the possibility of cryoglobulinaemia when the C4 is very low and we expect to find a few new cases every year through this route.
It is interesting to note that Sangur and colleagues recommend the investigation of cryofibrinogen in parallel with investigations for cryoglobulin. This is not our practice. Cryofibrinogenaemia is frequent in the background population (around 3% of blood donors), 2,3 and so selection on the basis of appropriate symptomatology is critical for the proper application of this assay.
It is clear, therefore, that there are still several areas of controversy to be resolved in the investigation of these difficult samples, but Sangur and colleagues have set a firm and sound platform for the debate. At the very least, the review by Sangur and colleagues challenges us all to revisit our practices for the assessment of cryoglobulinaemia if we are not to miss many cases of this important diagnosis. As they say ‘a badly performed cryoglobulin assay is often a waste of time, money and effort’.
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