Estimates of gas-phase abundances based on strong-line methods have been calibrated for H~{scshape ii} regions. Those methods ignore any contribution from the diffuse ionized gas (DIG), which shows enhanced collisional-to-recombination line ratios in comparison to H~{scshape ii} regions of the same metallicity. Applying strong line methods whilst ignoring the role of the DIG thus systematically overestimates metallicities. Using integral field spectroscopy data, we show how to correct for the DIG contribution and how it biases the mass--metallicity--star formation rate relation.
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