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Obtaining accurate coronary artery calcium (CAC) score measurements from CCTA datasets with virtual non-iodine (VNI) algorithms would reduce acquisition time and radiation dose. We aimed to assess the agreement of VNI-derived and conventional true non-contrast (TNC)-based CAC scores and to identify the predictors of accuracy. CCTA datasets were acquired with either 120 or 140 ​kVp. CAC scores and volumes were calculated from TNC and VNI images in 197 consecutive patients undergoing CCTA. CAC density score, mean volume/lesion, aortic Hounsfield units and standard deviations were then measured. Finally, percentage deviation (VNI - TNC/TNC∗100) of CTA-derived CAC scores from non-enhanced scans was calculated for each patient. Predictors (including anthropometric and acquisition parameters, as well as CAC characteristics) of the degree of discrepancy were evaluated using linear regression analysis. While the agreement between TNC and VNI was substantial (mean bias, 6.6; limits of agreement, 178.5/145.3), a non-negligible proportion of patients (36/197, 18.3%) were falsely reclassified as CAC score ​= ​0 on VNI. The use of higher tube voltage significantly decreased the percentage deviation relative to TNC-based values (β ​= ​−0.21 [95%CI: 0.38 to −0.03], p ​= ​0.020) and a higher CAC density score also proved to be an independent predictor of a smaller difference (β ​= ​−0.22 [95%CI: 0.37 to −0.07], p ​= ​0.006). The performance of VNI-based calcium scoring may be improved by increased tube voltage protocols, while the accuracy may be compromised for calcified lesions of lower density. The implementation of VNI in clinical routine, however, needs to be preceded by a solution for detecting smaller lesions as well. Acquiring accurate virtual non-iodine (VNI) images from coronary artery CT angiography datasets could decrease both acquisition time and radiation dose associated with routine calcium scoring. In the current paper, the in-vivo agreement of Agatston scores obtained with a novel spectral reconstruction VNI algorithm and with conventional true non-contrast (TNC) images were investigated and potential predictors of performance were identified. The investigation revealed that VNI-based calculations correlate significantly with TNC calcium scores, demonstrating substantial agreement. On the other hand, increased tube voltage protocols and higher calcium densities may enhance the performance of such algorithms, manifesting in reduced percentage deviation from TNC calculations. [ABSTRACT FROM AUTHOR]