In this Letter, a theoretical analysis and design methodology of integrated long period gratings (LPGs) for refractometric applications are proposed. A detailed parametric analysis is applied to a LPG model based on two strip waveguides to highlight the main design variables and their effect on the refractometric performances, with focus on the spectral sensitivity and signature response. To illustrate the proposed methodology, four variants of the same LPG design are simulated with eigenmode expansion, displaying a wide range of sensitivities up to 300,000 nm/RIU with figures of merit (FOMs) as high as 8000.