I'm not a specialist in this field, but it would appear to me that 'contracting laboratory' is a correct translation. See the reference below from the EMA site, for example. This pertains to sub-contacting, but still. For what it's worth, Van Dale renders 'loonbedrijf' as 'contracting firm'.

http://www.ema.europa.eu/docs/en_GB/document_library/Regulat...
6.6. Sub-contracting laboratory analysis If analysis or evaluation of clinical trial samples is sub-contracted to another laboratory, the ability of the sub-contractor to perform the work must be assessed prior to its initiation. Particular attention should be paid to staff training.
Before placing work with a sub-contractor, the sponsor, or their representative, should be informed and, if necessary, the contract with the sponsor amended.
A contract or service level agreement should be implemented between the two laboratories prior to the initiation of any work. Any such contract or service level agreement should clearly state roles anddelegated tasks and the scope and nature of the work that will be undertaken by the sub-contractor.
Care should be taken to ensure that contracts do not conflict with the requirements of the clinical trial protocol, work instruction or the contract between the analytical laboratory and the sponsor.