The ectoparasite test detects the presence of rickettsial antibodies in the patient’s serum by performing a nonspecific agglutination reaction with OXl9, OX2, and OXK, which have common bacteriophage antigens with rickettsiae. After the human body is infected by rickettsiae, the corresponding antibodies are gradually produced in the serum, which appear 5-12 days after the onset of the disease and basically disappear after several months, and generally the agglutination value is above 1:160 or the potency rises significantly during the course of the disease has diagnostic significance. So, how should the differential diagnosis of a negative test be made? The following is the differential diagnosis of the negative test: typhoid fever, typhus, anthrax, glandular plague, leptospirosis, etc. should be distinguished. 1, typhoid fever is slow onset, indifferent expression, a few rose rash, no scab ulcers, blood cultures have typhoid bacilli growth, positive Fester’s reaction, negative Fever’s reaction. 2, typhus is mostly seen in winter and spring, no scab and local lymph node enlargement, positive OX19 reaction, negative OXk, positive complement binding test with Przewalski’s or Rickettsiae as antigen. 3, leptospirosis gastrocnemius muscle pain is obvious, no scab, ulcer and rash. Leptospira can be found in the blood film. Leptospira complement binding test positive and positive latex agglutination test. 4, skin anthrax has a history of livestock contact, lesions are mostly seen in exposed areas, toxemia symptoms are mild, there is no rash, the total leukocyte count in the blood picture is more elevated, Bacillus anthracis can be detected by taking secretions, and the external Félix reaction is negative. Normal human titer (serum dilution times) does not exceed 1:20 (1) increased: epidemic typhus (OX19 positive rate can be 100%); endemic typhus (OX19 part up to 1:200-1:800); tsutsugamushi loyalists, the first week after the disease OXK has 14% in 1:80 or more. Up to 80% in the fourth week. (2) The titer in the serum of patients with brucellosis and regression fever is also increased. It may be slightly increased in pregnant women. Reference value: slide agglutination method: OX2 < 1:160OX19 < 1:160OXK < 1:160. Complement binding test: negative. Immunofluorescence staining method: negative.