The Netherlands national trial register identifier
https://www.who.int/ictrp/network/primary/en/
https://www.trialregister.nl/
NTR identifier
A centrally registered identifier that is assigned for a specific clinical trial registered in the The Netherlands national trial register (NTR). The format for the registry number is “NTR” followed by a 1 to 4-digit number, e.g., NL8498.
Clinical Trial Registry Identifier