Tourniquet time is the reason for this recommendation. The length of time that a tourniquet should be on for drawing all blood samples is no more than 1 minute. Lengthy tourniquet times cause changes in the venous endothelial that produces changes in the lab values for many tests. Longer tourniquet time is usually needed for insertion of a short peripheral catheter using customary techniques of seeing and feeling the vein.
Instead of drawing the sample as you insert the catheter, finish the procedure with complete securement and dressing. Then draw the sample. This allows for the tourniquet to be removed and the normal blood flow to flush away the blood containing the cellular substances that leads to alterations in lab values.
Learn more about the decisions for obtaining a blood sample in our online course – Blood Sampling: Venipuncture or VAD? The focus is now on preventing hospital-acquired anemia associated with blood sampling and several other issues.