A falsely high potassium or falsely low calcium value may occur due to which reason?

The correct answer emphasizes the sequence of blood collection and how it affects test results. When EDTA tubes are collected before serum tubes, the EDTA—a strong anticoagulant—can interfere with the measurement of certain analytes, including calcium. EDTA binds calcium ions, which results in a falsely low calcium value in the sample.

This is particularly important because serum tubes contain no anticoagulants and are designed to allow the blood to clot, enabling accurate measurement of serum levels of various biochemicals, including calcium and potassium. If a phlebotomist collects EDTA tubes first, the presence of EDTA in the blood sample can alter the results before the serum is able to be properly separated from the cells during the testing process.

The other options may involve incorrect practices in blood collection but do not directly lead to falsely high potassium or falsely low calcium values in the same way. For instance, using a tourniquet that is too loose would generally not cause significant changes in analyte levels, and sample contamination with heparin could lead to varying results but not specifically high potassium or low calcium. Not allowing blood to properly circulate affects blood flow and appearance but does not directly result in these specific falsely altered values.