NAT IM is for those students who have passed their FSc and A Levels in Pre Medical group. NAT IM is applicable in some universities for admission to BS or BSc programs(not for MBBS and BDS in medical colleges or medical universities. Check nts.org.pk for better details.). Whereas MCAT is Medical College Admission Test. Its score is applicable for admission to MBBS and BDS programs in medical universities and medical colleges (both private and government).
