Hi,

the widely-used method is based on H2O2 degradation rate.

use the following link:

http://www.sigmaaldrich.com/technical-documents/protocols/biology/enzymatic-assay-of-catalase.html

Note:

Use trusty and reliable brand for H2O2. You should know the exact purity, although you can measure it.

Serum should be clear with no suspended particles.

Avoid hemolysis.

Good luck