If the results of the Bethesda assay are positive, it means that there is a detectable level of antibodies working against the deficient coagulation factor being produced as a result of being treated with factor. The degree to which this affects a person is measured in "units." Inhibitors are classified into two categories based upon the highest unit level achieved. Those with a 5 or higher Bethesda unit result at any time are usually classified as having a "high responding" inhibitor level; those who measure below 5 units despite repeated exposure to factor concentrate, are classified as having a "low responding" inhibitor level.
People with high responding inhibitor levels often have quick and strong immune system responses directed against factor (VIII or IX), meaning that the inhibitor level can quickly increase to very high levels of antibodies. In some cases, if there is no further exposure to the replacement factor, the levels may drop over a period of months to years, even to an undetectable range. This does not mean that the inhibitor is gone, as it may reappear with further exposure, often times months later.
In cases where a person is said to have a "low responding" inhibitor level, the body's immune response to factor is slow--it produces a persistently low level of antibodies despite the person's continual exposure to factor concentrate.
A positive test result does not mean a person will always have an inhibitor. Treatment for inhibitors may cause them to disappear. Uncommonly, with the passing of time, inhibitors have spontaneously disappeared without treatment. These are often low responding inhibitors and are then classified as transient inhibitors.
Copyright National Hemophilia Foundation