To cite package 'ltm' in publications use:

Dimitris Rizopoulos (2006). ltm: An R package for Latent Variable Modelling and Item Response Theory Analyses, Journal of Statistical Software, 17 (5), 1-25. URL https://doi.org/10.18637/jss.v017.i05

Corresponding BibTeX entry:

  @Article{,
    title = {ltm: An R package for Latent Variable Modelling and Item
      Response Theory Analyses},
    author = {Dimitris Rizopoulos},
    journal = {Journal of Statistical Software},
    year = {2006},
    volume = {17},
    number = {5},
    pages = {1--25},
    url = {https://doi.org/10.18637/jss.v017.i05},
  }