Post-COVID syndrome (long covid, post COVID-19 condition) is characterized by cognitive and mental disorders, chest and joint pain, impaired sense of smell and taste, as well as by gastrointestinal and cardiac disorders. The diagnosis of post-COVID syndrome is based mainly on the patients' complaints. To date, no optimal diagnostic method has been proposed. The study was aimed to compare the informative value of the indicators obtained during conventional assessment of patients with post-COVID syndrome and the blood levels of neutrophil (NETs) and monocyte (METs) extracellular traps. The study involved neutropils and monocytes collected from 21 patients with post-COVID syndrome aged 18–59. Fluorescence microscopy and the SYBR Green (Evrogen) fluorescent dye for double-stranded DNA were used for enumeration and imaging of extracellular traps. Clinical and laboratory indicators make it impossible to identify the changes specific for post-COVID syndrome. At the same time, post-COVID syndrome is characterized by inflammation in the vascular endothelium. The filamentous forms of NETs found in blood are a laboratory feature of such aseptic inflammation. The filamentous forms of NETs have been detected only in those patients who have a history of mild to severe СOVID-19, while the filamentous forms of METs have been found in patients having a history of severe infection. The findings show that the detection of the filamentous forms of NETs and METs in blood is the most informative diagnostic feature of post-COVID syndrome.