This paper presents a novel configuration for Ion sensitive Field Effect Transistors (ISFETs) to be used as a threshold detector during a Single Nucleotide Polymorphism (SNP) base pair match. ISFET-based inverters are used as reaction threshold detectors to convey the chemical reaction level to a logic output once a threshold has been reached. Using this, DNA logic functions are derived for nucleotides allowing identification of multiple SNPs. This logic is extended to facilitate a combination of SNPs, deriving gates which give a simple Yes/No answer to their presence. A DNA logic `NOR' gate is derived for an application where two SNPs need to absent in order to determine drug delivery.