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Q fever - Diagnosis
Last reviewed: 03.07.2025
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Laboratory diagnostics of Q fever is based on serological methods: RA, RSK, RNIF, the results of which are analyzed taking into account the phase variations of Coxiella, which allows differentiating between patients and those who have recovered (standard diagnostics).
The simplest and most sensitive test - RA is used in macro- and micromodification. Agglutinins by the 8-10th day of the disease are detected in diagnostic titers of 1:8-1:16. Maximum titers (1:32-1:512) are noted by the 30-35th day of the disease. Then, gradually decreasing, they remain in the body of the patient from several months to several years.
In clinical practice, the CFR is most widely used. Detection of complement-fixing antibodies depends on the phase state of the corpuscular antigen of Burnet's coxiella used in the reaction. Antibodies to the second-phase antigen indicate an acute, "fresh" pathological process, appear from the 9th day of the disease and persist until 11-23 years, and antibodies of the first phase appear from the 30th day and persist for no more than 2-3 years. Detection of antibodies to both phase variants of coxiella indicates either a chronic form of the disease or an anamnestic nature of the reaction, and not the disease at a given time period. A high concentration of antibodies to the first-phase antigen indicates a chronic infection and is typical for patients with subacute or chronic coxiella endocarditis. Antibodies in the CFR are detected later than in RA. The highest titers (1:256-1:2048) are recorded on the 3-4th week from the onset of the disease. They persist for a long time - 3, 5, 7, 11 years. To differentiate markers of the acute process and "anamnestic" antibodies, a dynamic examination is necessary ("paired sera"); confirmation of the disease is an increase in titer by 2-4 times.
Recently, RNIF has been increasingly used, since antibodies in this reaction are detected earlier than in RA.
Thus, the diagnosis of Q fever is based on the identification of a complex of clinical, epidemiological and laboratory data.
[ Differential diagnosis of Q fever
Due to the polymorphism of symptoms, clinical diagnosis of Q fever is extremely difficult and is possible only in endemic foci in the presence of epidemic morbidity.
Differential diagnostics of Q fever is carried out with influenza, typhus and typhoid fever, brucellosis, ornithosis, pneumonia of various etiologies, anicteric leptospirosis, and sepsis.
In case of lung damage, it is necessary to differentiate the disease from tuberculosis (especially if the lesions are located in the upper parts of the lungs). In case of Q fever with scanty clinical symptoms, significant radiographic changes are possible already in the first days of the disease.
Flu differs from Q fever by a more acute onset and pronounced intoxication, the presence of muscle pain in the absence of joint pain, a short-term febrile reaction, persistent tracheitis, the absence of hepatosplenomegaly, and pronounced contagiousness.
Q fever is characterized by significant similarities with typhoid-paratyphoid diseases (gradual onset, prolonged fever, bradycardia, pulse dicrotia, tongue changes, hepatosplenomegaly, hemogram). It differs from typhoid fever by less pronounced toxicosis, almost constant absence of rash and positive Padalka symptom, less pronounced hepatosplenomegaly, earlier onset of typhoid status, negative results of serological and bacteriological examination.
Differential diagnostics of Q fever is carried out with chronic forms of brucellosis based on the characteristic damage to the locomotor system, nervous system, internal organs, genitourinary system and the presence of fibrositis in brucellosis.