Cell-based Antioxidant Protection in Erythrocytes (CAP-e) is a novel in vitro assay for antioxidant activity, developed and validated by NIS Labs specifically for the testing of natural products.  The CAP-e assay reflects antioxidant bioavailability at the cellular level.  The CAP-e assay is also a prerequisite for proper data interpretation of many other types of cell-based bioassays.

Antioxidant testing of natural products offers a wide range of testing options, but not all are biologically relevant. A strategic testing path is as follows:RBC in vessel

  • Demonstrate that antioxidants are present,
  • Evaluate whether antioxidants can enter into cells and protect cells from free radical damage, using the CAP-e assay,
  • Examine whether other bio-relevant effects may help reduce overall free radical burden, even in cases where a compound is not bioavailable at the cellular level,
  • Evaluate clinical effects by testing blood samples in a modified version of the CAP-e assay.

We specifically developed the CAP-e assay to help bridge the transition from chemical antioxidant assays (such as ORAC and total phenolics testing) to biological testing in more complex cellular systems.  Our CAP-e White Paper explains the development of the CAP-e assay and describes various applications in vitro and in vivo/ex vivo.


The CAP-e assay is performed by allowing red blood cells (erythrocytes) to absorb antioxidants from a physiological liquid that contains a potential antioxidant product. All compounds that are not absorbed into the cells are then removed. The cells are exposed to oxidative stress. An indicator dye shows the degree of intracellular damage to the cell, in the presence versus absence of serial dilutions of the test product.

The CAP-e assay has undergone extensive validation at NIS Labs, and lends itself to QA/QC application for many types of natural products.

Marketing claims pertaining to antioxidants should be specific, according to how the documentation of antioxidant capacity was performed.

What does the antioxidant capacity of a product mean? The answer depends on the environment in which it is tested.

  1. Antioxidant activity may be demonstrated in a given chemical reaction,
  2. Cellular antioxidant protection (CAP-e) data may show cellular bioavailability?
  3. Clinical data may show that a product’s antioxidants (or metabolites thereof) found their way into the blood stream.

Other CAP-e pages on this site: