Commitments and Contingencies - Narrative (Details) - USD ($)	12 Months Ended
	Apr. 30, 2021	Apr. 30, 2020
Commitments and Contingencies [Line Items]
Royalty expense	$ 127,000	$ 128,000
Minimum
Commitments and Contingencies [Line Items]
Royalty fee per tumor sample	$ 0
Royalty payment, as percent of contract price	2.00%
Maximum
Commitments and Contingencies [Line Items]
Royalty fee per tumor sample	$ 10,000
Royalty payment, as percent of contract price	12.50%