A 45-year-old male with a 10-year history of chronic liver disease is hospitalized for recurrent gingival bleeding, epistaxis, and purpura over the past 6 months. Physical exam: dusky complexion, palmar erythema and spider angiomas, scattered petechiae and ecchymoses on the skin. Liver not palpable below the costal margin, spleen palpable 4.5 cm below the costal margin. Ascites sign (-). Which of the following coagulation tests is most likely to show the earliest abnormality?