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Previous data have shown that the most prevalent, indirect plasma volume (PV) measurement technique, which utilises changes in haematocrit (Hct) and haemoglobin concentration ([Hb]), underestimates actual PV changes during immersion, when compared to a direct tracer-dilution method. An increase in the F-cell ratio (whole-body haematocrit (Hctw) to large-vessel haematocrit (Hctv) ratio) has been purported as a possible explanation, probably due to hydrostatic and thermally-mediated changes during water immersion. Previous investigators have not quantified the F-cell ratio during immersion. Therefore, this study sought to determine the effect of the F-cell ratio on the indirect method during both, thermoneutral and cold-water immersions. Seven healthy males were tested three times, seated upright in air (control: 21.2°C SD ±1.1), and during thermoneutral (34.5oC SD ±0.2) and cold-water immersion (18.6oC SD ±0.2), immersed to the third intercostal space for 60 min. Measurements during the immersion tests included PV (Evans blue dye column elution, Evans blue dye computer programme, and Hct [Hb]), red cell volume (RCV; sodium radiochromate), cardiac frequency (fc) and rectal temperature (Tre). Plasma volume during the control trial remained stable, and equivalent across the three tests. There was a hydrostatically-induced increase in PV during thermoneutral immersion, when determined by the Evans blue dye method (16.2%). However, the Hct/[Hb] calculation did not adequately reflect this change, and underestimated the relative PV change by 43%. In contrast, PV decreased during cold immersion when determined using the Evans blue dye method by 17.9% and the Hct/[Hb] calculation by 8.0%, respectively, representing a 52% underestimation by the latter method. There was a non-significant decline in RCV during both immersions. Furthermore, an increase (8.6%) and decrease (-14.4%) in blood volume (BV) was observed during thermoneutral and cold-water immersions, respectively. The decline in RCV during thermoneutral immersion attenuated the BV expansion. Despite the disparity between the PV methods, there was no increase in the F-cell ratio during either immersion. In contrast, there was a significant decline in the F-cell ratio during the control: air and thermoneutral immersion, which may indicate that other, undefined variables may impact on the stability of the red cell compartment. The current study is the first to show that the Hct/[Hb] method clearly underestimates PV changes during both thermoneutral and cold-water immersion. Furthermore, RCV was shown, for the first time, to decline during both immersions. However, the changes in the F-cell ratio during this study, did not account for the underestimation of PV change using the Hct/[Hb] method.