That's exactly what i did:
"Axis I-3: "&{Aftercare_Plan.disch_diag_1_3}&" "&{Aftercare_Plan.disch_diag_1_3_Desc} (this is a formula field used on the report).
isNull({Aftercare_Plan.disch_diag_1_3_Desc}) is used as you suggested, but the "Axis 1-3" still prints.