Blood supply can be assessed by a cool or discolored extremity, but this is a late sign. Decreased capillary refill is an early sign and can be discerned by squeezing the toes of the injured leg and counting how fast the blood refills the toe. An example would be to take your warm, pink fingertip and squeeze it and then let go. It should take 1-3 seconds for the pinkness to return to the fingertip; this is normal capillary refill. In the event of an injury, refill may be decreased to 4-5 seconds or longer. This test reveals vascular insufficiency (decreased blood supply), which may result in cell death if left untreated; in a worse-case scenario, vascular insufficiency can lead to loss of a limb. If you don't pass this test, go to the ER ASAP.
Nerve supply can be assessed by a decreased sensation to touch, numbness or tingling of the extremity. Nerves may be stretched, torn or compressed in an injury. The sooner the injury is treated, the more likely that symptoms from nerve trauma can be reversed. Nerve injuries demand ER evaluation ASAP.
It's easy to make the decision to go to an ER for an obvious bone fracture or significant injury. However, if you are questioning the need for medical attention for some injury, always err on the safe side. Pain can always be your guide to determine if you should to go to the ER. Blood supply and/or nerve compromise, on the other hand, always require a trip to the ER.