An Unexplained Hyperphosphatemia in an Hispanic Woman , a Diagnosis Challenge
Authors: Cuervo sierra Jorge , Arteaga Silva Juan Esteban , Buitrago-Naranjo Luis Eduardo , Castellanos-Olarte William , amorocho-Perez Ruben , Argote-Bravo Eduardo , Flores-Jimenez Juan Antonio 
Published: November 2021

Abstract
Introduction
Hyperphosphatemia could be a finding in renal failure, tumor lysis syndrome, hypoparathyroidism, and ingestion of exogenous phosphate. We present the case of a woman with hyperphosphatemia in which we finally diagnosed multiple myeloma, serum phosphate level achieved normal range after chemotherapy started. Although hyperphosphatemia in multiple myeloma is a very rare finding, there are some reports. The proposed mechanisms include a large amount of paraproteins that interferes with phosphorus level determination showing a false increased level. When samples have been deproteinated before analysis, serum phosphorus level can decrease obtaining a true level. This case reminds clinicians the importance of seeking additional causes for abnormal phosphorus levels before starting treatments that could lead to hypophosphatemia.
Case report
An hispanic woman at the 7th decade of life that only suffered from controlled hypothyroidism, who assisted with complaints of axial pain mainly in thoracic and lumbar segments. She denied neurological and B symptoms. She had a CT scan performed and was referred to the hematology service.On clinical examination we didn't find any neurological signs, no megalies and no palpable nodes. The patient brought a CT spine scan showing thoracic images suggestive of mets with collapse of 50% in T12-L1 and a mass in S2.With these findings we decided to start intrahospitalary studies for excluding malignancy.Her relevant laboratories were: creatinine 0,7, BUN 25, Hemoglobin 9 gr/dl VCM 102 WBC 3330 Granulocytes 55% Platelet 222.000, UN 10,7, albumin 2.8, Lactate dehydrogenase 425. A series of labs seeking secondary causes of malignancy was made including plasmatic cell neoplasie.The hepatic function was normal but an unusually high Phosphorus level was detected (19,6) with normal calcemia (8.98), the tumoral biomarkers were in normal range (CEA, CA 19-9, Ca 125, AFP) and an elevated B2 microglobulin (8) was found. Additional labs included bence jones protein and PTH, both in normal range. A new Thoracic CT described mosaic infiltrates and an extensive bone involvement suggestive of lytic metastatic changes. The abdomen CT showed multiple lytic lesions in lumbo-sacral spine and pelvis, The skull radiography had lytic lesions too. The nephrology service was required by the Hyperphosphatemia and they asked about external intake of phosphorus finding that she was taking "hydrolyzed collagen", so they started iv fluid and chelators (aluminium hydroxide and no calcium chelators).Because of unresponsive hyperphosphatemia, nephrology started hemodialysis.Hematology found high IgG levels (5500) concluding the need for bone marrow studies . Finally a the bone marrow study was performed and prescribed steroids on suspicion of Multiple myeloma. Until this day the Phosphorus level still remained in high levels (17,7) despite hemodialysis sessions. The bone marrow study revealed a plasmatic cell population of 28,9% with big and abundant cytoplasm, some of them with binucleations suggestive of infiltration by plasmatic cell neoplasie. The seric electrophoresis revealed a paraprotein (seric protein of 9,3 with a monoclonal peak of gamma region of 3,4 gr/dl), the seric immunofixation was positive for a monoclonal IgG lambda component. With this report we began chemotherapy treatment for Myeloma with CyBorD regimen (Cyclophosphamide, Bortezomib, dexamethasone). After treatment was initiated, phosphorus level began to lower and achieved normal range (4.28) in the first two weeks. Then she was discharged to continue outpatient management. Nowadays she is receiving her 3rd chemotherapy cycle without complications, her phosphorus level still remains within normality.
Conclusions
Spurious hyperphosphatemia in patients with paraproteinemias like multiple myeloma can occur. The IgG monoclonal can interfere with the phosphomolybdate principle used in the assay of serum phosphate on most automated chemistry analysers. For labs which do not have a multilayered film technology based system, a serial dilution analysis of the suspected analyte could be performed.

Disclosures
No relevant conflicts of interest to declare.