Wednesday, November 23, 2011

Clinical case 47: Answers and Summary


Paraproteins may cause abnormal laboratory findings in 3 ways: 1) through the disease 
process itself; 2) by interacting with the target of an assay; 3) by creating spurious results due to their 
interference with the assay method. Paraproteins have been shown to cause interference with the 
assays of multiple laboratory tests, including blood counts, sodium, calcium, phosphorus, lipids, 
coagulation profiles, iron studies, blood urea nitrogen, creatinine, bilirubin, c-reactive protein, 
glucose, uric acid, lactate dehydrogenase, alkaline phosphatase. 

All the above listed findings have been described with paraproteins except the hypermagnesemia.  Obviously, the most commonly discussed on board questions is pseudohyponatremia but that we don't see that often. Given the burden of light chains, usually one now encounters the others.  See the references below for full primary data on these.  IgM, given its size, is usually the big culprit but igG and igA have also been described.  Paraproteins exert biphasic interference on serum phosphate levels, usually resulting in factitious hyperphosphatemia but occasionally in hypophosphatemia. Pseudohyperphosphatemia may be seen in patients with IgM, IgA, or IgG paraproteins. In addition, hypophosphatemic patients showing falsely “high” levels of phosphate may be labeled as pseudonormophosphatemic, and this may be just as clinically relevant as pseudohyperphosphatemia. Deproteination can usually correct this artifact.

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Pseudohyponatremia, pseudohypochloridemia, and reduced anion gap have also been described when using the indirect ion-specific electrode method.  Pseudohypercalcemia due to an IgM paraprotein has been reported in two cases in the literature.
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