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Cover - shares
 9 Months Ended
Sep. 30, 2021
Nov. 05, 2021
Cover [Abstract]    
Document Type 10-Q  
Document Quarterly Report true  
Document Period End Date Sep. 30, 2021  
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 Address, Address Line One 1261 Liberty Way  
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 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 Common Stock, Shares Outstanding (in shares)   16,164,232
Entity Central Index Key 0001274737  
Current Fiscal Year End Date --12-31  
Document Fiscal Year Focus 2021  
Document Fiscal Period Focus Q3  
Amendment Flag false