LAUSR

Introduction: Phosphorus is an essential component of many macromolecules found in bone, lipid membranes, and DNA. It circulates in serum as phosphate. Phosphate level is mainly determined by kidney function. Other factors such as 1, 25 vitamin D3, thyroid hormone and low phosphorous intake can increase renal absorption of phosphate. Hyperphosphatemia presents when serum phosphate is above 4.5 mg/dl (1.45 mmol/L). The phosphate target for hemodialysis (HD) patients is 5.5 mg/dl (1.77 mmol/L) or less. Serum phosphate is commonly measured through the colorimetric method and can also be measured isotopically. Depending on the method used to measure the serum phosphate, many factors have been reported to produce falsely elevated levels. Methods: 52-year-old female with past medical history of end stage renal disease on HD, heart failure with severely reduced ejection fraction secondary to ischemic cardiomyopathy status post left ventricular assist device (LVAD), type 2 diabetes, hypertension, upper gastrointestinal bleeding, anemia, was admitted at a rehabilitation center after a hospital stay due to COVID-19 infection and E. faecium bacteremia secondary to a drive-line infection of the LVAD which was treated with daptomycin. On admission, the patient was found to have a phosphorus level of 6.3 mg/dl, PTH 265 pg/mL, corrected calcium 10 mg/dl, and hemoglobin 9.1 g/dL. Results: Patient was started on oral Sevelamer tablets 800 mg every 8 hours and underwent regular full HD sessions. However, the hyperphosphatemia persisted. Sevelamer was increased to 1600 mg every 8 hours, and she was maintained on a strict low phosphorous renal diet. Four days later while on the new regimen, the phosphorous increased to 12.2 mg/dl. She remained asymptomatic. Hemolysis and hyperbilirubinemia were excluded. A serum protein electrophoresis revealed a monoclonal spike in the gamma region with gamma % of 38.5 (normal range 11-20), gamma globulin 3.0 g/dL (normal range 0.6 – 1.6 g/dL), and quantification of the abnormal protein of 0.41 g/dL (5.3% total). Serum immunofixation showed a probable IgG Lambda monoclonal band. A serum free light chain assay demonstrated a Kappa light chain free serum of 548.9 mg/L (normal range 3.3 – 19.4 mg/L) and Lambda light chain free serum 549.7 mg/L (normal range 5.7 – 26.3 mg/L). Patient was diagnosed with a monoclonal gammopathy, and the elevated phosphorus deemed to be pseudohyperphosphatemia secondary to paraproteinemia. Conclusions: Colorimetric assay with phosphomolybdate ultraviolet (UV) is commonly used for measurement of serum phosphate. The ammonium molybdate reacts with the phosphate to form a cloudy complex, UV absorbance is measured at a specific wavelength. Several factors have been reported to cause falsely high phosphate such as hyperlipidemia, hyperbilirubinemia, hemolysis, liposomal amphotericin B, recombinant tissue plasminogen activator, heparin sulfate, and gammopathies. The paraproteinemia present in monoclonal gammopathies creates a cloudier sample which increases the absorbance of UV light leading to spurious elevation of serum phosphate. Although hyperphosphatemia is a common finding in dialysis patients, the presence of persisting or worsening hyperphosphatemia in a compliant patient taking phosphorous binders and adhering to a low phosphorus diet should raise concern for pseudohyperphosphatemia. No conflict of interest