Cryoglobulins are named as such because they are antibodies which precipitate in the cold. In order to test for circulating cryos, a blood specimen must be drawn and immediately placed at 37 degrees. Once the blood has clotted (the test cannot be performed for patients on heparin), the sample is then centrifuged at 37 degrees, and the serum is placed at 4 degrees. Every day, the sample is checked for for turbidity; once cloudy the sample is centrifuged to give a quantitative value to the “cryocrit.” Some cryoglobulins take up to 5 days to precipitate.
The “poor man’s cryoglobulin” test–and faster in terms of turnaround time–is a rheumatoid factor with complement levels (C3 and C4 are both typically low in cryoglobulinemia).