Ionized calcium

The free, biologically active form of calcium in blood. During alkalosis, more calcium binds to albumin, so ionized calcium temporarily falls and nerves and muscles may become more excitable. Total calcium in a blood test may look normal even when the free fraction has dropped for a short time. This temporary dip is a key reason for tingling and cramping during fast breathing. It normalises as CO₂ and pH return to their usual range after the breathing phase.