Which condition will cause erroneous Cai results?

The condition that leads to erroneous calcium (Cai) results is the assay of whole blood collected in sodium oxalate. Sodium oxalate is an anticoagulant that binds calcium, making it unavailable for measurement. In the context of calcium assays, this results in a lower measured value since the calcium that is present is effectively sequestered by the sodium oxalate. Therefore, using sodium oxalate as an anticoagulant disrupts the analysis of calcium levels in the blood.

Conversely, the other options, while they might affect overall results, do not directly interfere with the measurement of calcium in the same manner. For example, slight hemolysis during venipuncture might lead to the release of intracellular contents but is less likely to significantly impact calcium measurements compared to the binding effect of sodium oxalate. Analysis of serum stored at 4°C might impact various biochemical parameters but typically does not alter the calcium concentration due to temperature alone. Lastly, whole blood collected in sodium heparin generally preserves the calcium levels as heparin does not bind calcium in the same way that oxalate does. Thus, sodium oxalate has a unique and direct impact on the accuracy of calcium assays, leading to erroneous results.