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Organization and Description of Business - Additional Information (Detail)
 1 Months Ended 3 Months Ended
Feb. 11, 2021
USD ($)
shares
Jan. 25, 2021
USD ($)
$ / shares
shares
Jun. 15, 2020
USD ($)
$ / shares
shares
Apr. 30, 2020
USD ($)
Mar. 31, 2021
USD ($)
uniqueSolution
center
phlebotomist
patient
Dec. 31, 2020
USD ($)
Jan. 31, 2018
patient
Schedule of Capitalization, Equity [Line Items]              
Number of renal transplant patients (more than) | patient             1,700
Number of transplant centers offering RemoTraC (more than) | center         200    
Number of patients enrolled in RemoTraC (more than) | patient         7,000    
Number of mobile phlebotomists (more than) | phlebotomist         10,000    
Accumulated deficit         $ 353,214,000 $ 352,527,000  
Cash, cash equivalents, and short-term investments         374,300,000    
Accelerated and advanced payment       $ 20,500,000      
Grants receivable         $ 30,000,000,000.0    
Medicare fee-for-service reimbursements proceeds, initial tranche       $ 4,800,000      
Public Offering              
Schedule of Capitalization, Equity [Line Items]              
Shares issued and sold (in shares) | shares   1,923,077 4,492,187        
Sale of stock (in usd per share) | $ / shares   $ 91.00 $ 32.00        
Sale of stock, consideration received on transaction   $ 164,000,000.0 $ 134,600,000        
Over-Allotment Option              
Schedule of Capitalization, Equity [Line Items]              
Shares issued and sold (in shares) | shares 288,461            
Sale of stock, consideration received on transaction $ 24,700,000            
XynManagement, Inc.              
Schedule of Capitalization, Equity [Line Items]              
Number of unique solutions | uniqueSolution         2    
Medicare | AlloSure Kidney              
Schedule of Capitalization, Equity [Line Items]              
Reimbursement rate         $ 2,841    
Medicare | AlloMap Heart              
Schedule of Capitalization, Equity [Line Items]              
Reimbursement rate         3,240    
Medicare | AlloSure Heart              
Schedule of Capitalization, Equity [Line Items]              
Reimbursement rate         $ 2,753