Arsenic

A metalloid found in groundwater in some regions, in some foods, and in a few industrial settings.

Long-term exposure through drinking water is a recognised cause of skin and internal disease in affected areas.

Seafood contains a harmless organic form in quantity, so a meal of fish or shellfish in the previous two days can raise total arsenic several-fold with no exposure to the toxic form -- which is why speciated testing exists and why diet before the draw matters more here than almost anywhere.

Worth discussing a raised value, and worth saying what you ate beforehand.