Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (373 page)

   
Coagulation
: Thrombin time is prolonged due to inhibition of fibrin polymerization by the paraprotein (impaired coagulation may play a role in the bleeding diathesis).

   
Serum
β
-2 microglobulin
is elevated in half the patients.

   
Sedimentation rate
and
C-reactive protein
may be very elevated.

   
LDH and alkaline phosphatase
when elevated correlate with an unfavorable course.

   
Hyperuricemia and hypercalcemia
have been reported.

   
Azotemia
may be present on the basis of light chain or amyloid depositions, as well as parenchymal renal involvement by lymphoplasmacytic cells.

   
Tests no longer recommended

   Immunoelectrophoresis (replaced by immunofixation).

   BJ protein may be replaced by measuring serum light chains because the amount of IgM excreted in urine may be below detection level and does not correlate well with tumor burden. In addition, obtaining serum light chain analysis obviates the need to collect 24-hour urine.

   Limitations

   Spurious results: The high-level IgM may interfere with automated analyzer results, especially producing an artificially low HDL cholesterol or falsely elevated Hgb.

   Serum IgM may occasionally be artifactually low because of the polymerization of IgM. A warm bath collection should be obtained for blood specimens in patients suspected of having cryoglobulinemia to avoid underestimation of serum IgM.

   Low serum ferritin levels may be the result of interference of the IgM paraprotein with ferritin measurements.