When collecting blood sample from vein, It is easier to enter a vein which can be felt than the one that is only seen.
The techniques should not be applied to tightly not too long another not to cause venustus is week and leave the concentrations of certain blood substance such as hemoglobin, plasma proteins and calcium.
It is not proper to collect blood for chemical analysis from the arm into which and intravenous infusions is already being given.