This study is a non-intervention, clinical study to test "research use only" APAP protein adducts dipsticks in human blood samples. The study will be conducted in two phases. In phase 1, the research use only dipsticks and dipstick testing components will be tested for feasibility in the emergency department at UAMS. Patients will be enrolled that are having an acetaminophen level obtained as part of their medical evaluation. In phase 2, the research use only dipsticks will be tested in centers that participate in the Acute Liver Failure Study Group (ALFSG), a clinical research network funded by the National Institutes of Diabetes, Digestive and Kidney Diseases (NIDDK). Subjects participating in the clinical registry of the ALFSG will be approached for enrollment. These patients all have the diagnosis of acute liver failure or acute liver injury from a variety of etiologies, including APAP. A single blood sample will be obtained and will be aliquoted for 1) testing by the dipstick and 2) testing by high performance liquid chromatography with electrochemical detection (HPLC-EC) analysis for APAP protein adducts. APAP levels will also be determined from the second aliquot of blood. In both Phase 1 and Phase 2, the dipsticks will be read manually and by dipstick readers by independent testers that are not directly involved in the clinical care of the patients. The results of the two dipstick readings will be recorded, will remain confidential and will not be used in the diagnosis or management of the patient. The results of the dipsticks and HPLC-EC assays will be compared to the final patient diagnoses.