Many reports have detailed the interaction of blood proteins with endotoxin making detection with the LAL assay, or alternatives such as rFC or endotoxin-specific ELISA, very difficult.  A recent report from New Zealand addressed this directly and found that standard kinetic assay protocols produce inaccurate results in serum samples.  Additionally, they found complications with various Vacutainer tubes, owing to the anti-coagulant used and/or the tube material:

The Limulus Amebocyte Lysate assay may be unsuitable for detecting endotoxin in blood of healthy female subjects

The scientists in this report conclude the paper with the observation that a suitable method for freeing the sequestered endotoxin from the blood proteins is essential.  BioDtech offers solutions to remove endotoxin masking for both protein samples (using EndoPrep) and blood products (using ESP).

BioDtech, Inc. offers many products to assist in both removal and detection of endotoxin. 

EndoBind-R™ is an affinity chromatography media designed to remove endotoxin from aqueous solutions.  The ligand is highly specific for endotoxin resulting in high detoxification and low product loss.

EndoPrep™ is a sample preparation kit used in conjunction with LAL or rFC assays.  It removes the interfering effects of proteins that can mask endotoxin or inhibit enzymatic assays thus providing more accurate  endotoxin quantitation.