International normalized ratio (INR) monitoring is essential during oral anticoagulation therapy to minimize bleeding complications and thrombotic events. Laboratory technicians cannot differentiate falsely elevated INR results due to problems in drawing blood from truly elevated INR values, but the extent of this phenomenon is unclear. We determined the proportion of patients with INR values of more than 9 in our laboratory who had values less than 6 in the hospital when retested on the same day. There were 251 tests with an INR of more than 9 (1.84 per 1,000 tests), and 188 of them (74.9%) had an INR of 10 or more, or 1.37 per 1,000 tests sent. There were 20 (33%) of 61 patients with INR values of more than 9 in our laboratory with repeated test values in the hospital of less than 6. Efforts are warranted to decrease this proportion, and patients and physicians should be aware of this phenomenon to prevent dissatisfaction owing to inappropriate expectations.