A simple, precise, and sensitive method is described for the determination of ionised calcium in plasma at normal body temperature and pH, using an ultrafiltrate of the plasma. This technique is based on a two-wave-length (480 nm, 550 nm) spectrophotometric method, using pH independent dye ammonium tetramethylmurexide (ATM). By this method, the normal (mean + 2 S.D.) plasma ionised calcium in 19 normal subjects is 1.04 + 0.14 mmol/l which is similar to some of those reported for ion-selective electrode measurements.
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