Unexplained fever is a difficult disease in internal medicine, the etiology is complex and involves many systemic lesions, the etiological diagnosis of FUO has always been a worldwide problem, so far there are still nearly 10% of patients with “unexplained fever” can not ultimately clarify the cause of the disease, today, combined with a little personal experience of diagnosing FUO for many years, the diagnostic thinking and methods of FUO. This is a brief introduction to the diagnostic thinking and methods of FUO. First, the definition of FUO: refers to the fever lasts more than 3 weeks, the temperature exceeds 38.3 ℃ several times, after a week of detailed history, physical examination and routine laboratory tests can not confirm the diagnosis. It excludes numerous short-term fevers of easier etiologic diagnosis and unspecified fevers that resolve spontaneously within a short period of time, and it also eliminates functional fevers that manifest as low-grade fever. The incidence of true FUO is not high, but the etiology is complex and the diagnosis is tricky, making it a test for both clinicians and patients. Second, the etiology of FUO classification: domestic and foreign research shows that there are more than 200 kinds of FUO etiology, mainly infectious diseases, rheumatic diseases, malignant neoplastic diseases (including hematologic malignant tumors), other diseases and diagnosis of unknown diseases of five categories. Infectious diseases account for 30%-50%, with tuberculosis, sepsis, abdominal abscess, endocarditis, typhoid fever, brucellosis, fungal infections, cytomegalovirus infections, etc.; rheumatic diseases account for 20%-30%, mainly adult Still’s disease, systemic lupus erythematosus, Wegener’s granulomatosis, unclassified connective tissue disease, desiccation syndrome, systemic vasculitis, tuberculosis, aortitis, rheumatoid artery disease, giant cell arteritis and other diseases. Giant cell arteritis, rheumatic polymyalgia, seborrheic membrane inflammation, dermatomyositis, etc.; malignant neoplastic diseases accounted for 10-20%, commonly malignant lymphoma, malignant histiocytosis, myelodysplastic syndromes, leukemia, renal cell carcinoma, hepatocellular carcinoma, lung cancer, intestinal cancer, etc.; other diseases accounted for 5-10%, of which drug fever, necrotizing proliferative lymphadenopathy, Crohn’s disease, hemangioblastoid lymphadenopathy are common; diagnosis of unknown diseases about about the same. Other diseases account for 5-10%, among which drug fever, necrotizing proliferative lymphadenopathy, clonal disease, angioimmunoblastic lymphadenopathy are the most common ones. Third, the diagnosis of FUO thinking and methods: 1, a comprehensive and detailed understanding of the medical history, especially for the cause of fever to provide clues to the relevant history, including: ① fever history: such as fever onset time, fever, fever, fever; ② fever pattern: whether accompanied by chills, rhythmic, whether accompanied by other specific symptoms (such as rashes, arthralgia), whether there is a seasonal; ③ history of disease: such as tuberculosis, immune deficiency-related diseases; ④ special regional and regional fever, fever, fever, fever, fever, and other diseases. History of disease: e.g. tuberculosis, immunocompromise-related diseases; ④ History of settlement or traveling in special areas (infected areas, pastoral areas), history of contact; ⑤ Other: e.g. history of surgery, history of medication, history of travel, and so on. History inquiry is helpful in determining the etiology of a large proportion of FUO. 2, careful, meticulous, comprehensive physical examination, including any part of the body, any suspicious signs, especially need to pay attention to some special signs: such as skin rashes, hemorrhages, lymph nodes, liver, spleen enlargement, joint enlargement, deformity, dysfunction, localized elevation, mass, new heart murmur, lung rhonchi, vascular murmur, localized pressure pain, tenderness, and so on, the comprehensive physical examination may provide clues to the etiology of FUO. The comprehensive physical examination may provide clues for the diagnosis of FUO; pay attention to the parts that are easy to neglect, such as the oral cavity, throat, thyroid gland, finger tips, etc. 3.”Scattering “the combination of auxiliary examination and targeted special examination, when the history and signs can not provide clues; based on “experience” can not “speculate” results; has been made of the examination When there is no doubt, “casting a net” auxiliary examination is necessary, such as the detection of blood sedimentation, CRP, calcitoninogen, multi-site imaging, repeated blood cultures, a variety of autoimmune indexes, a variety of tumor markers, multi-site, multiple bone marrow puncture, organ puncture, etc.; in the “casting a net” combination of special examinations. At the same time of “casting a net” examination, patients with certain clues should be subjected to certain targeted examinations, such as biopsy of enlarged lymph nodes, biopsy of skin, muscle, blood vessels, superficial parts of the mass, pericardial, thoracic, abdominal, joint fluid, cerebrospinal fluid puncture, and some of the examinations need to be repeated many times, so that further examinations should not be abandoned just because of the negative results of one or two times of examination. Some tests even need to be repeated several times, and further tests should not be abandoned because of one or two negative results. Fourth, some special means of FUO etiologic diagnosis When repeated examinations, consultations, still difficult to clearly diagnose, it is necessary to take some special practices. Such as: stopping observation, in order to exclude or determine whether it is “drug fever”; trial of antibacterial drugs, highly suspected bacterial infection of patients, in the repeated culture of body fluids and find the site of infection at the same time, you can choose the appropriate antibiotics for experimental treatment, and through the observation of the therapeutic effect of assisting in the diagnosis of a number of patients through experimental anti-infective treatment to get cured. Anti-infection treatment is cured, while the cause of the disease has never been clarified; trial of anti-tuberculosis drugs, in the careful exclusion of other diseases, and at the same time can not be excluded tuberculosis, experimental anti-tuberculosis treatment can be considered, in general, the use of the drug after 2 to 3 weeks of the body temperature will have a relatively significant reduction, otherwise the possibility of tuberculosis should be suspected, and turned to other diagnostic considerations. Immunosuppressants, when highly suspected autoimmune disease, or malignant lymphoma, while not supporting infectious diseases, can consider experimental treatment with glucocorticosteroids, but with enough experience and sufficient inclination, not rash, so as not to cause serious consequences; nutritional supportive therapy, long-term fever, especially hyperthermia, should pay attention to nutritional supportive therapy, to prevent because of the one-sidedly focusing on the search of fever Nutritional supportive therapy should be emphasized for patients with prolonged fever, especially high fever, in order to prevent them from focusing on finding the cause of fever and neglecting the supportive therapy, which will make the patient fail before the cause is clear and lose the time of diagnosis and treatment. FUO etiology diagnosis experience: 1, seriously, step by step, avoid subjective, preconceived notions, easy to draw conclusions, but also to calm down, stabilize the spirit, do a good job of the patient and the family work, explaining the complexity of the etiology of FUO diagnosis and difficulty, to obtain understanding and cooperation. 2, multidisciplinary consultation is necessary, with the theory and experience of experts in related fields, it is helpful to clarify the etiology of the diagnosis, but the invitation to consultation should be purposeful, the physicians involved in the consultation should have rich clinical experience in internal medicine, especially experience in diagnosis and treatment of FUO, it is best to designate the consultation experts and departments, through the consultation of doctors with special experience, it will help to solve the practical problems, improve the quality of consultation, invite the pathological and diagnostic imaging experts to participate in the consultation is also very important. It is also meaningful to invite pathology and diagnostic imaging experts to participate in the consultation. Avoid abusing antibiotics, glucocorticosteroids and non-steroidal antipyretics during the waiting period. Such as the abuse of antibiotics and glucocorticoids may induce secondary infections, especially fungal infections, or produce drug fever and other side effects of drugs, thus masking the original appearance of the disease, aggravating and complicating the condition; or inducing drug resistance, increasing the difficulty of follow-up treatment; there is also the possibility of inducing the recurrence of old tuberculosis, etc.; the application of non-steroidal anti-inflammatory and antipyretic drugs will affect the observation of temperature, delaying the diagnosis, and the large amount of sweating when the heat subsides will also lead to defecation, electrolyte disorders, antipyretics also have induced drug fever and hematopoietic disorders, skin rashes, gastrointestinal bleeding, gastrointestinal reactions and other side effects. FUO is a difficult and complicated disease often faced by internists, the etiology of which involves hundreds of diseases in multiple disciplines and specialties, requiring clinicians to not only have the right way of thinking when dealing with “unexplained fever”, but also have a solid knowledge of clinical theory and rich clinical experience, especially in the diagnosis and treatment of infectious diseases, rheumatic diseases, blood and blood circulation disorders. Infectious diseases, rheumatic diseases, hematological and neoplastic diseases account for the majority of the causes of FUO, amounting to more than 80%, because infectious diseases, rheumatic diseases, hematological and neoplastic diseases are the causes of FUO.