I think your logic flow is flawed - Indent the code and it becomes more obvious;
Code:
If rs.AbsolutePosition > 0 Then
If rs!LastReadingID > 0 And rs!LastDoseID > 0 Then rs!Trend = GetGlucoseStatement(rs!SugarID, rs!LastReadingID, rs!LastDoseID, "Both")
ElseIf rs!LastReadingID > 0 Then rs!Trend = GetGlucoseStatement(rs!SugarID, rs!LastReadingID, rs!LastDoseID, "Reading")
ElseIf rs!LastDoseID > 0 Then rs!Trend = GetGlucoseStatement(rs!SugarID, rs!LastReadingID, rs!LastDoseID, "Dose")
End If
You may find this easier with a Select Case statement - they are certainly a lot easier to read.