Is a discard tube necessary, when drawing blood for P-Ionized calcium analysis?

Area of Science:

• Clinical chemistry
• Preanalytical variation studies

Background:

• Preanalytical variation significantly impacts laboratory test accuracy and diagnostic precision.
• Venous blood collection procedures, including the use of discard tubes, can influence test results.
• The specific impact of discard tubes and storage duration on ionized calcium measurements requires investigation.

Purpose of the Study:

• To evaluate the necessity of using a discard tube in venous blood collection for ionized calcium analysis.
• To assess the effect of varying storage durations on the stability of ionized calcium measurements.
• To determine the clinical relevance of any observed differences in ionized calcium levels due to collection method or storage.

Main Methods:

• Collected 100 paired venous blood samples using a winged collection system without a discard tube.
• Stored samples at 4-6°C for 24, 48, 72, 96, and 120 hours.
• Analyzed P-Ionized Calcium (pH adjusted 7.4) using a Konelab 60i and compared results between the first and second tubes using Bland-Altman plots and Wilcoxon's rank-sum test.

Main Results:

• No statistically significant differences were observed between the first and second tubes across all 100 samples.
• A statistically significant difference in P-Ionized Calcium was detected in samples stored for 120 hours (p=0.0068).
• The observed difference in 120-hour stored samples was not considered clinically relevant.

Conclusions:

• The use of a discard tube is not required for accurate P-Ionized Calcium (pH adjusted 7.4) measurements.
• While prolonged storage (120 hours) can lead to statistically significant changes, these are not clinically significant.
• Standard blood collection protocols for ionized calcium appear robust regarding the discard tube practice.