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Hemoconcentration occurs when fluid filters out of the vein while cellular and protein components remain, artificially increasing analyte concentrations (e.g., protein, calcium, potassium). It is commonly caused by prolonged tourniquet application and vigorous fist pumping. Best practice is to apply the tourniquet just long enough to locate and access the vein, release it within one minute, and release it before withdrawing the needle once blood flow is established in the final tube. Option C reflects this approach. Option A promotes hemoconcentration and should be avoided. Option B can raise potassium, lactate, and other values due to muscle activity and fluid shifts; patients may make a gentle fist but should not pump. Option D pertains to needle selection; gauge choice should match vein size and device, but it does not specifically address hemoconcentration. Additional tips include selecting an appropriate site, ensuring patient hydration when possible, and avoiding draws above IV infusions or through hematomas, all of which affect specimen integrity. Controlling preanalytical variables is a core competency for reliable laboratory results.