There are many possible variants that could be used here, including "certificate of fitness for work", "fitness for work report", "occupational fitness report/certificate/evaluation". I think that for "carta" either "certificate" or "report" would be the best words, and either would do. "Certificate" is somewhat more common". Or indeed "assessment" or "evaluation" could be used, but I would reserve them for the act of assessing the worker's fitness.

"Fitness for work", for which there are a number of synonyms used (fitness for duty, fitness to work, occupational fitness, fitness for employment, fitness for task, job fitness) is probably the most common general term. The reference to LGSS art. 196 shows that this is primarily about medical fitness to do a particular job:

"3. Las indicadas empresas no podrán contratar trabajadores que en el reconocimiento médico no hayan sido calificados como aptos para desempeñar los puestos de trabajo de las mismas de que se trate. Igual prohibición se establece respecto a la continuación del trabajador en su puesto de trabajo cuando no se mantenga la declaración de aptitud en los reconocimientos sucesivos."
http://noticias.juridicas.com/base_datos/Admin/rdleg1-1994.t...

"Criteria and methods used for the assessment of fitness for work: a systematic review"
Assessment of fitness for work is defined by most as the evaluation of a worker's capacity to work without risk to their own or others' health and safety. It is mainly assessed at recruitment (pre‐offer or post‐offer), and when changes of work or health conditions occur. [...]
In some countries, the certificate of fitness for work is equivalent to the candidate's acceptance in a pension plan or a company's private medical insurance."
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2092557/