To cite package 'ltm' in publications use:

Rizopoulos D (2006). “ltm: An R package for Latent Variable Modelling and Item Response Theory Analyses.” Journal of Statistical Software, 17(5), 1–25. 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},
  }