I have been POMO for ten + years now. MANY years ago "Blood cards" were shown to obtain a couple of internal publications at conventions. In the 90s we received some letters from Watchtower for elders to physically check publishers' blood cards to make sure they were "up to date".
Looking back, this was a way to essentially "out" anyone who wasn't a "true believer".
What better way to prove your loyalty to a cult than a suicide pact card in your wallet 24/7?
Due to HIPA regulations in the USA now, patients can simply request any hospital to not release their name or hospital room. This does not prevent JWs who work in the hospital (billing, records, nursing, etc.) from telling local elders about any patients taking blood, having STDs, having drug addictions, or getting abortions. (Not uncommon).