Objective
Precise measurements of fluid status lack valid methods. Bioimpedance is an attractive diagnostic tool because it is noninvasive, quick, and relatively cheap. This systematic review aims to assess the existing evidence of bioimpedance as an accurate measure of fluid status in critically ill patients.

Data sources
PubMed and Embase up till March 2021 were systematically searched (PROSPERO: CRD42020157436).

Study selection
Eligibility criteria were studies reporting original data from cohorts of adult patients in intensive care units and doing at least one whole-body bioimpedance and one reference test. In addition, studies assessing internal reproducibility were included.

Data extraction
An extraction form was designed for the purpose.

Data synthesis
Nine hundred five studies were screened for eligibility, and 28 studies, comprising 1482 individual patients, were included in the final analysis. Eight studies compared bioimpedance with a gold standard, and two of those reported the results adequate. We found a low mean difference, but the 95% limits of agreements had wide limits. The remaining studies applied different surrogates as reference tests. Correlations ranged from 0.05 to 0.99. The Grading of Recommendations Assessment, Development and Evaluation (GRADE) certainty of evidence for all outcomes was very low.

Conclusions
The accuracy of bioimpedance as a measure for fluids in critically ill patients in the intensive care unit cannot be determined. Due to the lack of a gold standard, numerous studies compared bioimpedance with surrogate outcomes with great variability in both designs and results. Assessing the internal reproducibility of bioimpedance had the same limitations, but the studies overall reported good internal reproducibility.