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Cover - USD ($)
 12 Months Ended
Dec. 31, 2019
Mar. 20, 2020
Sep. 30, 2019
Cover [Abstract]      
Document Type 10-K    
Document Annual Report true    
Document Period End Date Dec. 31, 2019    
Document Transition Report false    
Entity File Number 001-39049    
Entity Registrant Name EXAGEN INC.    
Entity Incorporation, State or Country Code DE    
Entity Tax Identification Number 20-0434866    
Entity Central Index Key 0001274737    
Current Fiscal Year End Date --12-31    
Document Fiscal Year Focus 2019    
Document Fiscal Period Focus FY    
Amendment Flag false    
Entity Address, Address Line One 1261 Liberty Way,    
Entity Address, Address Line Two Suite C    
Entity Address, City or Town Vista    
Entity Address, State or Province CA    
Entity Address, Postal Zip Code 92081    
City Area Code (760)    
Local Phone Number 560-1501    
Title of 12(b) Security Common Stock, par value $0.001 per share    
Trading Symbol XGN    
Security Exchange Name NASDAQ    
Entity Well-known Seasoned Issuer No    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Interactive Data Current Yes    
Entity Filer Category Non-accelerated Filer    
Entity Small Business true    
Entity Emerging Growth Company true    
Entity Ex Transition Period false    
Entity Shell Company false    
Entity Public Float     $ 104,900,000
Documents Incorporated by Reference Certain information required to be disclosed in Part III of this report is incorporated by reference from the registrant's definitive Proxy Statement for the 2020 Annual Meeting of Stockholders, which proxy statement will be filed not later than 120 days after the end of the fiscal year covered by this Form 10-K.    
Entity Common Stock, Shares Outstanding   12,627,056