Final answer:
The factor that causes a false decrease in PCV is shortened centrifuge time, as it does not allow sufficient time for RBCs to settle at the bottom of the tube leading to a falsely low reading.
Step-by-step explanation:
You have asked which factor causes a false decrease in packed cell volume (PCV). Among the options provided, the one that can cause a false decrease in PCV is 'shortened centrifuge time'. PCV, also known as hematocrit, measures the percentage of red blood cells (RBCs) in a blood sample. When a blood sample is not centrifuged for the appropriate amount of time, the RBCs do not have sufficient time to fully settle at the bottom of the tube, resulting in a falsely low PCV reading.
In contrast, a low blood-to-anticoagulant ratio may result in clotting, which could falsely increase PCV. Icterus (jaundice) does not affect PCV but can interfere with other blood test measurements. Clots in the blood sample can also falsely increase the PCV as they cause RBCs to be trapped within the clot.