Adsorption of serum calcium by plastic sample cups

Area of Science:

• Clinical Chemistry
• Biochemistry
• Analytical Chemistry

Background:

• Accurate serum calcium measurement is critical for diagnosing various medical conditions.
• Plastic sample cups used in automated analyzers can potentially interfere with analyte stability.
• Previous studies have not fully elucidated the mechanism of calcium loss in automated sample handling.

Purpose of the Study:

• To investigate the phenomenon of low calcium values observed in sera stored in AutoAnalyzer cups.
• To identify the factors contributing to calcium adsorption onto plastic sample cup walls.
• To provide recommendations for preventing calcium measurement errors in clinical laboratories.

Main Methods:

• Sera were incubated overnight in AutoAnalyzer sample cups (polystyrene and polypropylene).
• Serum pH changes and subsequent calcium adsorption were monitored.
• The effect of different plastic materials and storage conditions on calcium levels was evaluated.

Main Results:

• Sera left in plastic AutoAnalyzer cups showed reduced calcium values, attributed to adsorption.
• Increased serum pH significantly promoted calcium adsorption onto cup walls.
• Polystyrene cups exhibited greater calcium adsorption compared to polypropylene cups.
• As much as 10% of serum calcium could be adsorbed, posing a practical concern.

Conclusions:

• Serum calcium analysis is susceptible to errors from adsorption onto plastic AutoAnalyzer cups, exacerbated by pH increases.
• Prompt analysis of fresh serum or refrigerated storage at 4°C is recommended to minimize adsorption.
• Avoid leaving serum in AutoAnalyzer cups at room temperature for extended periods (over 3 hours).