Welcome @creigstad,

According to this study "Validation of blood phosphatidylethanol as an alcohol consumption biomarker in patients with chronic liver disease" https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4409698/, blood transfusion could potentially affect the results of PEth testing.

See this excerpt:
"Large volume blood transfusions could result in false positive or false negative results for PEth testing, as may have been the case for one of our study participants. It is important to note that this was not necessarily a representative sample of patients with liver disease, as recruitment was completed in a regional referral center for liver diseases."

You may wish to discuss with your medical team.