Optical platelet aggregation (OPA) with platelet-rich plasma (PRP) was compared with a Thrombelastograph (TEG) whole blood assay for monitoring arachidonic acid (AA)-induced platelet activation. Assays were performed on 47 interventional cardiology and 24 general surgery patients receiving aspirin therapy for cardiovascular disease, as well as 48 volunteers asked to take nonsteroidal anti-inflammatory drugs (NSAIDs) or 12 volunteers on chronic NSAID therapy unrelated to diagnosed cardiovascular disease. Whole blood TEG monitoring of NSAID inhibition detected NSAID-insensitive AA activation of platelets in a significantly higher number of cardiology (23%) and surgery (25%) patients and normal volunteers on chronic NSAID (25%) therapy relative to normal subjects not on chronic NSAID therapy (0%). Whole blood NSAID insensitivity was observed with cyclooxygenase-I inhibitors, such as aspirin and ibuprofen; was not affected by Celebrex, a cyclooxygenase-II inhibitor; but was completely inhibited by thromboxane-receptor antagonists. This was not due to platelet NSAID insensitivity, because complete inhibition of AA-activation responses in PRP was observed with either TEG or OPA assays. We confirmed that thromboxane B(2) formation in PRP from NSAID-insensitive subjects was completely inhibited by NSAIDs. However, significant amounts were formed in whole blood from NSAID-insensitive subjects, but not in whole blood from NSAID-sensitive subjects. Thromboxane formation after AA addition was not found in washed blood cells with 90% reduced platelet counts or in leukocyte-rich buffy coat fractions, but could be restored by addition of PRP. NSAID-insensitive activation was inhibited by nordihydroguaiaretic acid, with an IC(50) of 30 micromol, implicating 12- and/or 15-lipoxygenases in this transcellular pathway.