Try this --
SELECT Patient_Table.*
FROM Patient_Table LEFT JOIN LabTest ON Patient_Table.PatientID =
LabTest.PatientID
WHERE Patient_Table.PatientID <> (SELECT [XX].PatientID FROM LabTest AS [XX]
WHERE [XX].Lab_Date >= DateAdd("m", -6, Date()) AND Patient_Table.PatientID =
[XX].PatientID)
ORDER BY Patient_Table.PatientID;
--
Build a little, test a little.