I don't do Access, but I didn't see a min or Max functions, and I don't understand DMin and DMax, but these functions will act as Min and as Max functions
Private Function Cap(Limit As Double, Value As Double) As Double
If Limit > Value Then
Cap = Value
Else
Cap = Limit
End If
End Function
Private Function LowerLimit(Lowest As Double, Value As Double) As Double
If Lowest < Value Then
LowerLimit = Value
Else
LowerLimit = Lowest
End If
End Function
Example usage in your code:
Dim NewCrea, NewTbil, NewINR
NewCrea = LowerLimit(1, Me.Creatinine) 'Also for NewTbil, NewINR
NewCrea = Cap(4, NewCrea )
LnCrea = Log(NewCrea) 'Also for LnTbil, LnINR
Me.Meld = Cap(40, ((0.957 * LnCrea) + (0.378 * LnTbil) + (1.12 * LnINR) + 0.643) * 10