Determination of ionized calcium in parotid saliva

A new method for the determination of ionized calcium in parotid saliva utilizing an automatic ionized calcium analyzer (Orion Biomedical Model Space-Stat 20) was evaluated. The method showed good precision with a relative standard deviation of 2.9--3.8 per cent in the range 0.20--0.79 mmol . l-1 after 226 duplicate measurements. Mean recovery was 94.9% in the range of 0.6--1.8 mmol. l-1. Bovine mucin was found not to interfere with the calcium electrode. Differences in sodium concentration, ionic strength and pH between the standard solutions and samples caused systematic errors. These were corrected using experimentally derived formulas. The influence of pH on the concentration of ionized calcium was studied in 13 parotid saliva samples. A non-linear relationship ranging between 32 and 84% of the total calcium concentration was found.