Is paenia or penia correct English?

Answer 1

Penia is correct in both UK and US English.

In UK English, words describing blood and using the word "haem" to build their prefix or suffix are spelled with an "ae" - e.g. haemophilia, leukaemia. Similarly with paediatrics. Haem is a Latinised form of the Greek haima, which means blood. Paed is a Latinised form of the Greek pais, meaning child.

Penia is a Greek word meaning insufficiency or a lack of. It has been taken straight from the Greek into our medical terminology. Some do Latinise it and use paenia, but this would only really make sense if it were painia in Greek.