The concentration of free calcium ions in capillary blood from neonates on a routine basis using the ICA 1

Abstract

The concentration of free calcium ions (C_Ca²⁺), and pH was measured in 200 full-term infants aged 5-130 h and in 50 healthy adults aged 18-60 years with a new semi-automatic electrode system (ICA 1 Radiometer, Copenhagen) in 110 μ1 capillary whole blood. The blood was obtained anaerobically by heel puncture of the infants and from the earlobe of the adults. No bias was found for the erythrocyte effect on the liquid junction potential with a haematocrit below 0.60 but for a haematocrit between 0.60 and 0.80 there was an increasing negative bias of 1-5% for this instrument.

The special heparinized glass capillaries (Radiometer, Copenhagen) used for blood sampling were studied in serum pools at different levels of C_Ca²⁺ and the anticoagulant used was shown to give a bias of less than ± % on the measurement of C_Ca²⁺ in the range 0.862.02 mmol/l.

In full-term infants aged 548 h C_Ca²⁺ was 1.271±0.098 mmol/l (±2 SD) and pH 7.42±0.07 (±2 SD). In infants aged 49-96 h, C_Ca²⁺ was 1.334±0.136 mmol/l (±2 SD) and pH 7.42±0.07 (±2 SD). In infants, aged 97-130 h C_Ca²⁺ was 1.383±0.140 mmol/l (±2 SD) and pH 7.40±0.076 (±2 SD).

In capillary blood from healthy adults, aged 18–60 years C_Ca²⁺ was 1.2820.07 mmol/1 (±2 SD) and pH 7.42±0.03 (±2 SD).

The analytical standard deviation for C_Ca²⁺ in capillary blood determined from 400 measurements on duplicately drawn capillaries from newborn infants was 0.019 mmol/l and 0.024 for actual pH. Day to day analytical standard deviation for C_Ca²⁺ at pH 7.40 was 0.016 mmol/l for an equilibrated donor serum pool used as an internal quality control.

Key Words:

• capillary blood sampling
• liquid junction potential
• paediatric
• quality control
• reference values