Cover - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Mar. 12, 2021	Jun. 30, 2020
Cover [Abstract]
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2020
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 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
ICFR Auditor Attestation Flag	false
Entity Shell Company	false
Entity Public Float			$ 83.4
Entity Common Stock, Shares Outstanding		12,669,816
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 2021 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 Central Index Key	0001274737
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	FY
Amendment Flag	false