Among the United Nations' 17 Sustainable Development Goals (SDGs) launched in 2015, the fourth goal (SDG 4) is dedicated to education, and one of the ten targets within that goal specifically addresses adult literacy and numeracy skills. Efforts to reach this target involve monitoring, which in turn involves assessment. The most powerful instrument for assessing literacy proficiency is the Programme for the International Assessment of Adult Competencies (PIAAC), conducted by the Organisation for Economic Co-operation and Development (OECD). It has five hierarchically organised proficiency levels for literacy. A sixth category, labelled "below Level 1", lumps together low proficiencies at the bottom end of the proficiency continuum. To boost effective action in addressing SDG 4, the UNESCO Institute for Statistics (UIS) recently launched the Global Alliance to Monitor Learning (GAML), which aims to support national assessment strategies and to develop internationally comparable indicators and methodological measurement tools. While PIAAC Levels 1-5 are already broadly suitable for international comparison, the "below Level 1" category has so far only been assessed by individual countries (e.g. Canada, the United States, the United Kingdom and Germany) using instruments developed nationally. Focusing on the reading aspect of literacy, the authors of this article investigate how these nationally developed low proficiency assessment instruments might be adjusted to facilitate international comparability.