Requirements for collection and processing of blood specimens for cryoglobulin testing
• Blood is collected into prewarmed tubes. The sample is then allowed to clot for 1 hour, followed by centrifugation and separation of the serum. All of these steps, including transportation of the sample after collection, must be performed at 37°C. Premature cooling may decrease the cryoglobulin concentration and result in false-negative results.
• The remaining serum is incubated at 4°C (refrigerator temperature) for 3 to 7 days. A 7-day incubation period is ideal because many mixed cryoglobulins will not be identified otherwise due to slow precipitation.
• Centrifugation at 4°C is performed. Visual inspection then allows for determination of the presence of a cryoprecipitate, which, if present, indicates a positive result (qualitative screen). The use of specialized Wintrobe tubes can allow positive results to be measured as a percentage, the cryocrit (quantitative measure). Some laboratories only report one of these two measurements, with no further testing performed.
• If a cryoprecipitate is present, further testing should be requested to phenotype the constituents of the precipitate. In this analysis, the sample is washed to remove any potential contaminates. The precipitate is then rewarmed and the contents of the cryoglobulin (specifically the Ig isotypes and assessment for clonality) and quantity can be determined by electrophoresis and immunofixation. Some laboratories may use nephelometry for Ig quantification. These tests allow for classification of the cryoprecipitate according to the Brouet system
