What is the difference between the direct coombs test and indirect coombs test?

Bound vs unbound ABs: Direct coombs' test is used to detect antibodies that are already bound to the surface of red blood cells. Many diseases and drugs (quinidine, methyldopa, and procainamide) can lead to production of these antibodies indirect coombs' test looks for unbound circulating antibodies against a series of standardized red blood cells. The indirect coombs' test is only rarely used to diagnose a condition.