The OECD is planning to enhance existing PISA assessment instruments in reading, mathematics and science so that they will be more suitable to the context of developing countries. The main purpose of this paper is to identify the main technical issues associated with this aim. The paper reports detailed analysis of the existing PISA item pool and its suitability for countries which have students of average limited capacity. The paper cautions that the fit of developing country data to the PISA model is not good and that modifications to address some of the deviations should be explored. The use of learning metrics to describe dimensions of educational progression is at the core of the PISA reporting methodology and requires a consistency across countries in item behaviour that is not apparent for developing countries. The paper recommends that any process to move towards enhancing the instruments must be undertaken with extensive consultation with the countries involved.