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