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1. Antibody can be prepared against cAMP and polysacchrides, not just protein.
2. During sample preparation, you do need to add inhibitor of cAMP phosphodiesterase, such as 3-Isobutyl-1-methylxanthine (IBMX, a non-specific inhibitor of cAMP or cGMP phosphodiesterase).
3. ELISA is expensive and may not be sensitive, you can try radioimmunoassay.