Acetaminophen (APAP) level
Obtain 4 hrs post-ingestion and compare level to Rumack-Matthew nomogram
Obtaining level between 2-4 hrs post ingestion is helpful if level is undetectable (essentially zero risk of progressing to toxic levels), but anything other than undetectable requires a repeat draw at 4 hrs
Obtaining multiple levels is rarely indicated in the absence of hepatotoxicity
Chemistry
Metabolic acidosis seen with extremely large ingestion
Lactic acid and blood gas
LFT
AST and ALT elevation more specific than GGT and alkaline phosphatase
AST levels > 1000 IU/L are more likely to result from acetaminophen poisoning than from chronic hepatitis or alcoholic liver disease
PT/PTT/INR
Aspirin levels and other co-ingestants
Obtain 4 hrs post-ingestion and compare level to Rumack-Matthew nomogram
Obtaining level between 2-4 hrs post ingestion is helpful if level is undetectable (essentially zero risk of progressing to toxic levels), but anything other than undetectable requires a repeat draw at 4 hrs
Obtaining multiple levels is rarely indicated in the absence of hepatotoxicity
Metabolic acidosis seen with extremely large ingestion
Lactic acid and blood gas
AST and ALT elevation more specific than GGT and alkaline phosphatase
AST levels > 1000 IU/L are more likely to result from acetaminophen poisoning than from chronic hepatitis or alcoholic liver disease
Only indicated for single, acute ingestion occurring <24 hr prior to presentation
Not useful for chronic ingestion (patients who take supratherapeutic doses for several days) or if time of ingestion is unknown
Make sure you use the correct units!
Dotted line should be used for those at higher-risk of liver toxicity (eg alcoholics, those on enzyme-inducing drugs)
Co-ingestion of drugs that reduce GI motility should prompt repeating acetaminophen level at 8 hrs:
Opiates
Anticholinergics (diphenhydramine, etc.)
Not useful for chronic ingestion (patients who take supratherapeutic doses for several days) or if time of ingestion is unknown
Opiates
Anticholinergics (diphenhydramine, etc.)
Acetaminophen toxicity
Acetaminophen toxicity 
Mescaline toxicity
Viral hepatitis
Cocaine toxicity
Hepatitis A
Carbamate toxicity
Hepatitis B