A client who received 6 units of packed red blood cells 3 days ago for a lower gastrointestinal (GI) bleed is now displaying signs of shortness of breath with occasional stridor and is reporting muscle cramping. Reference Range: Potassium [3.5 to 5 mEq/L (3.5 to 5 mmol/L)] Magnesium [Adult: 1.3 to 2.1 mEq/L (0.65 to 1.05 mmol/L)] Calcium [9 to 10.5 mg/dL (2.3 to 2.6 mmol/L)] Sodium (136 to 145 mEq/L (136 to 145 mmol/L)] Which serum laboratory value should the nurse immediately report to the healthcare provider?