How much of an effect will loratadine have on panel testing if inadvertently taken within a day or two of beginning the test?

Not much: if anything. By panel testing, I presume you mean standard blood tests measuring usual chemistries done for a check-up. If by panel testing you mean allergy skin tests, the recommendation is to avoid antihistamines for a full 48 hours prior to testing as the drug may diminish otherwise positive skin test reactions. Drugs of abuse tests should not be affected. I hope this helps. Stay well:)