Commitments and Contingencies - Narrative (Details) - USD ($)	3 Months Ended	12 Months Ended
	Jul. 31, 2016	Apr. 30, 2018	Apr. 30, 2017
Commitments and Contingencies [Line Items]
Operating leases, rent expense		$ 657,000	$ 398,000
Corporate Headquarters
Commitments and Contingencies [Line Items]
Operating leases, rent expense	$ 86,000	90,000
Laboratories and Office Space
Commitments and Contingencies [Line Items]
Operating leases, rent expense		59,000	105,000
New York Laboratory
Commitments and Contingencies [Line Items]
Operating leases, rent expense		52,000	207,000
Rockville, MD
Commitments and Contingencies [Line Items]
Operating leases, rent expense		$ 454,000	$ 0
Minimum
Commitments and Contingencies [Line Items]
Royalty payment, as percent of contract price		2.00%
Maximum
Commitments and Contingencies [Line Items]
Royalty fee per tumor sample		$ 7,000
Royalty payment, as percent of contract price		5.00%