Leeches are generally useful on replanted digits, ring avulsion injuries and in small free flaps where there is good arterial inflow but no venous outflow. Before the decision to use leeches is made, however, it is recommended that consideration be given to other methods for improving circulation, such as operative revision of microvascular anastomoses, removal of sutures to relieve tension, etc.

It is very important that leeches only be used in conditions of venous congestion with good arterial inflow. Leeches will not be helpful in cases of insufficient arterial inflow. Insufficient arterial supply could lead to infection from any source, including the leech. The following criteria may help in diagnosing a true venous problem in a flap:

• Skin Color...........Dusky or bluish
• Capillary Return.....Brisker than normal (note that areas of fixed coloration are beyond salvage)
• Pinprick Response....Bleeding should be rapid and dark
• History..............Known problems with veins at operation, either in the pedicle or at the site of a microvascular anastomosis