Red cell effects on lithium measurements by ion-selective electrode

Area of Science:

• Clinical Chemistry
• Analytical Chemistry
• Biomedical Instrumentation

Background:

• Accurate lithium measurement is crucial for therapeutic drug monitoring.
• The NOVA 11 analyzer is used for determining lithium levels.
• Haematocrit variations can potentially affect analytical results.

Purpose of the Study:

• To evaluate the analytical performance of the NOVA 11 analyzer for lithium.
• To investigate the interference of haematocrit on lithium measurements.
• To develop a method for accurate lithium determination in whole blood.

Main Methods:

• Assessed within-run and day-to-day imprecision using quality control solutions.
• Compared whole blood and plasma lithium measurements across a range of haematocrits.
• Quantified red blood cell interference by varying haematocrit in lithium-free samples.
• Analyzed patient samples and spiked samples to assess haematocrit and lithium concentration effects.

Main Results:

• Within-run imprecision <1.06% and day-to-day imprecision <2.8% for lithium (0.56–0.68 mmol/L).
• Satisfactory correlation between whole blood and plasma lithium measurements (deviations up to 0.06 mmol/L for 35-53% haematocrit).
• Identified and quantified a specific red blood cell interference.
• Observed that apparent lithium levels decrease as haematocrit increases after accounting for red cell effects.

Conclusions:

• Haematocrit significantly interferes with lithium measurements on the NOVA 11 analyzer.
• A specific red blood cell interference effect was identified and quantified.
• An algorithm can be developed to correct for haematocrit interference, enabling accurate whole blood lithium analysis.