Differential diagnosis apparatus adapted for medical applications in order to determine an optimal sequence of diagnostic tests for identifying a pathology by adopting diagnostic appropriateness criteria, comprising a system of processors, in turn comprising: - a first updatable database containing patients' data; - a second updatable relational database containing identification data of pathologies, symptoms, clinical signs, identification data of diagnostic tests, and data relating to the appropriateness parameters of said diagnostic tests for defining a list of diagnostic hypotheses (pathologies); - means adapted to determine said optimal sequence of diagnostic tests for identifying a pathology, said means comprising an inferential computation engine, which determine for each diagnostic hypothesis (pathology), based on data contained in said first and second databases, said optimal sequence of diagnostic tests with associated global and single indexes of appropriateness and probability that a patient is suffering from that pathology, said differential diagnosis apparatus being characterized in that said means are adapted to: - determine said optimal sequence of diagnostic tests; - determine said appropriateness index IA of a single diagnostic test defined as: IA = To / (C * Te * R), where Te is the test access wait time index; C is the test cost index; R is the index of the maximum between the intrinsic risk and the relative risk of the test; To is the test tolerability index; - determine said global appropriateness index of a sequence of diagnostic tests IA G =To G /(C G *Te G *R G), where To G is the minimum among all tolerability indexes of all tests in the sequence, C G is the sum of the single costs of each test in the sequence, Te G is the maximum among all wait times, R G is the maximum among all risk indexes of the sequence, wherein tolerability indexes reflect the tolerability of each test for the patient. - a first updatable database containing patients' data; - a second updatable relational database containing identification data of pathologies, symptoms, clinical signs, identification data of diagnostic tests, and data relating to the appropriateness parameters of said diagnostic tests for defining a list of diagnostic hypotheses (pathologies); - means adapted to determine said optimal sequence of diagnostic tests for identifying a pathology, said means comprising an inferential computation engine, which determine for each diagnostic hypothesis (pathology), based on data contained in said first and second databases, said optimal sequence of diagnostic tests with associated global and single indexes of appropriateness and probability that a patient is suffering from that pathology, said differential diagnosis apparatus being characterized in that said means are adapted to: - determine said optimal sequence of diagnostic tests; - determine said appropriateness index IA of a single diagnostic test defined as: IA = To / (C * Te * R), where Te is the test access wait time index; C is the test cost index; R is the index of the maximum between the intrinsic risk and the relative risk of the test; To is the test tolerability index; - determine said global appropriateness index of a sequence of diagnostic tests IA G =To G /(C G *Te G *R G), where To G is the minimum among all tolerability indexes of all tests in the sequence, C G is the sum of the single costs of each test in the sequence, Te G is the maximum among all wait times, R G is the maximum among all risk indexes of the sequence, wherein tolerability indexes reflect the tolerability of each test for the patient. - determine said optimal sequence of diagnostic tests; - determine said appropriateness index IA of a single diagnostic test defined as: IA = To / (C * Te * R), where Te is the test access wait time index; C is the test cost index; R is the index of the maximum between the intrinsic risk and the relative risk of the test; To is the test tolerability index; - determine said global appropriateness index of a sequence of diagnostic tests IA G =To G /(C G *Te G *R G), where To G is the minimum among all tolerability indexes of all tests in the sequence, C G is the sum of the single costs of each test in the sequence, Te G is the maximum among all wait times, R G is the maximum among all risk indexes of the sequence,