Objective Knowledge of intensive care unit (ICU) acquired hypernatremia (ICU-AH) has been hampered by the absence of granular data and confounded by variable definitions and inclusion criteria.

Design Multicentre retrospective cohort study.

Setting Twelve ICUs in Queensland (QLD), Australia.

Participants Adult patients admitted to ICU from 2015 to 2021. Only the first ICU admission was considered, and we categorised patients into mild (146–150 mmol·L−1), moderate (151–155 mmol·L−1) and severe (>155 mmol·L−1) ICU-acquired hypernatremia.

Main outcome measure We aimed to study the prevalence of ICU-AH, patient characteristics, trajectory, risk factors, and outcomes.

Results Data from 55,255 ICU admissions were included in the analysis, of which 4146 (7.5 %) patients had ICU-AH. These were subcategorised into mild (n = 2,670, 4.8 %), moderate (n = 1,073, 1.9 %) and severe (n = 403, 0.73 %) forms. Median time to diagnosis was 4 (2–6) d after ICU admission, while median time to peak serum sodium level was 5 (3–8) d. The median maximum sodium level across the cohort was 149 (147–152) mmol·L−1. The sodium correction rate was 1 mmol·L−1 per day, taking a median of 3 d (1–5) for sodium levels to return below 145 mmol·L−1. APACHE III score, invasive ventilation, fever, and diuretic use on the day before hypernatremia were independent risk factors for moderate or severe ICU-AH. After adjusting for confounders, all levels of hypernatremia were independently associated with an increased risk of 30-d in-hospital mortality.

Conclusions In a large multicentric study of critically ill patients, ICU-acquired hypernatremia occurred in one in eight admissions after a median of four days in the ICU and was preceded by identifiable and modifiable risk factors. If severe, its correction was slow, and normalisation was delayed. After adjusting for other factors, all levels of hypernatremia were an independent risk factor for 30-d in-hospital mortality.