A FXI assay yields different values at different dilutions: 1:10 23%; 1:20 42%; 1:140 80%. This pattern is most consistent with which reason?

A FXI assay yields different values at different dilutions: 1:10 23%; 1:20 42%; 1:140 80%. This pattern is most consistent with which reason?

• A. Clot
• B. Factor deficiency
• C. Inhibitor
• D. Bad draw

Answer: (C)

The pattern shows dilution-dependent restoration of activity, which points to an inhibitor present in the sample. When there is an inhibitor against Factor XI, the inhibitor binds the factor and suppresses the assay result. At higher concentrations (lower dilutions), the inhibitor is more effective, so the measured activity is low. As you dilute the sample, there are fewer inhibitor molecules relative to Factor XI, so less inhibition occurs and more FXI activity is detected. That’s why the values rise from 23% at 1:10 to 42% at 1:20 and up to 80% at 1:140. A true deficiency would not improve with dilution in this way, and bad draw or clotting issues wouldn’t produce this consistent, dilution-dependent pattern.

The pattern shows dilution-dependent restoration of activity, which points to an inhibitor present in the sample. When there is an inhibitor against Factor XI, the inhibitor binds the factor and suppresses the assay result. At higher concentrations (lower dilutions), the inhibitor is more effective, so the measured activity is low. As you dilute the sample, there are fewer inhibitor molecules relative to Factor XI, so less inhibition occurs and more FXI activity is detected. That’s why the values rise from 23% at 1:10 to 42% at 1:20 and up to 80% at 1:140. A true deficiency would not improve with dilution in this way, and bad draw or clotting issues wouldn’t produce this consistent, dilution-dependent pattern.