Quantum Key Distribution (QKD) is a technology that enables the sharing of secret cryptographic keys between two distant users (Alice and Bob), with intrinsic security guaranteed by the fundamental laws of nature. QKD has become a mature technology, and in Europe, all 27 member states are collaborating on a European Commission initiative (EuroQCI) to design, develop, and deploy a quantum communication infrastructure. In Italy, the QUID project is responsible for implementing the Italian segment of EuroQCI. QKD relies on single photons to secure the distribution of the keys and, to become a viable real-world solution, the metrological characterization of optical components and systems is fundamental. To obtain the appropriate security requirements, test and evaluation methods at single-photon level need to be developed; in particular, since the single-photon detectors represent the most vulnerable part of a QKD system, their characterization in terms of operating parameters (quantum efficiency, dead time, jitter, afterpulsing..) is of the utmost importance. We present the INRIM efforts in the quantum efficiency calibration of single-photon avalanche detectors (SPADs), focusing on QKD application. The detection efficiency is evaluated for a fibre-coupled InGaAs/InP-SPAD and for a free-space Si-SPAD. The calibration is performed using different experimental setups and reference standards with proper traceability chains at the wavelength of 1550 nm and 850 nm respectively. Dependence of detection efficiency on polarization in superconducting nanowire single-photon detectors (SNSPDs) is also reported. The work is fundamental to align the Italian deployment of QKD, in the framework of QUID, with validation needs, providing test services for the characterization, validation and certification for QKD.