IMHO równie dobrze możesz przetłumaczyć dosłownie "test progowy" jeśli dalej masz wytłumaczenie co to jest. Polski system różni się od angielskiego i raczej nie będzie 100% odpowiednika, stąd poszedłbym w opisowość


http://www.cps.gov.uk/victims_witnesses/threshold.html
6. The Threshold Test
The Threshold Test requires Crown Prosecutors to decide whether there is at least a reasonable suspicion that the suspect has committed an offence, and if there is, whether it is in the public interest to charge that suspect.
The Threshold Test is applied to those cases in which it would not be appropriate to release a suspect on bail after charge, but the evidence to apply the Full Code Test is not yet available.
There are statutory limits that restrict the time a suspect may remain in police custody before a decision has to be made whether to charge or release the suspect. There will be cases where the suspect in custody presents a substantial bail risk if released, but much of the evidence may not be available at the time the charging decision has to be made. Crown Prosecutors will apply the Threshold Test to such cases for a limited period.
The evidential decision in each case will require consideration of a number of factors including:
the evidence available at the time;
the likelihood and nature of further evidence being obtained;
the reasonableness for believing that evidence will become available;
the time it will take to gather that evidence and the steps being taken to do so;
the impact the expected evidence will have on the case;
the charges that the evidence will support.
The public interest means the same as under the Full Code Test, but will be based on the information available at the time of charge which will often be limited.
A decision to charge and withhold bail must be kept under review. The evidence gathered must be regularly assessed to ensure the charge is still appropriate and that continued objection to bail is justified. The Full Code Test must be applied as soon as reasonably practicable.