Like giving someone a favorite star, only this function has a dual purpose. With the badge only being visible to the account holder that enabled it, another option would be made available under the sexual health tab. "I've Screened Positive"(or other wording) button would be available to all account holders. Once pressed, a check box screen should open (some that should not have the functionality to be unchecked, to decrease potential false/revenge action) and a save button. Once the account holder updates this status, every account that the use had enabled the "we fucked" badge for, within a specific time frame, would receive an anonymous or system generated direct message suggesting they get tested.
This badge can also assist in preventing those embarassing late night messages stating that you actually HAD previously met before, and their name definitely isn't Taylor.

Without criticism or negativity directed towards anyone, Grindr has actively assisted in the spread of STIs, (we all know the risk going in to it, so this is not an attempt to place blame) but I believe this is a function that should have been in place ages ago, for all accounts, paid or not.