APTT in tolerized hemophilia B dogs. The APTT values from the 5 tolerized dogs are listed as described in Figure 1 and show some shortening while on subcutaneous rhFIX prophylaxis. Horizontal lines mark the APTT range for healthy dogs (22-30 seconds) and for untreated hemophilia B dogs (70-90 seconds). Maintenance with subcutaneous rhFIX was started at day 112 as described in Table 1 and adjusted to 82.6 IU/kg twice weekly at day 178. ^* denotes a sample drawn during a rhFIX-free washout period. The APTT is less sensitive than the WBCT to low levels of plasma FIX. All samples were drawn just prior to the next dose. Error bars indicate SD.