The F-cell ratio: a clinically important parameter or just fine tuning?

Abstract

Prof. James P. Isbister, Head, Department of Haematology Royal North Shore, Hospital of Sydney, St. Leonards, NSW Australia 2065, Adjunct Professor, School of Biological Sciences, University of Technology, Sydney Determination of the venous haematocrit is one of the simplest and oldest tests in laboratory medicine. However, its interpretation from a physiological and pathophysiological point of view remains controversial and is frequently associated with dogma and confusion. For most of this century it has been repeatedly questioned as to what is the true relationship between the haematocrit of blood obtained from the macro-circulation and that of the circulation as a whole [1,2]. What variation is there in haematocrit between the macroand microcirculation and between different organs of the body, and do these relationships change with alterations in blood volume, red cell mass or plasma volume, and is the relationship altered by disease states? It is now generally accepted that the haematocrit of blood in macrocirculation is different from that obtained when the red cell mass and plasma volume are measured independently (body haematocrit). In the stable state in a normal person the relationship of the body haematocrit to the venous haematocrit is approximately 0.9, this is termed the F-cell ratio. This means that the venous haematocrit is 10% higher than that of the body haematocrit. The reasons for this have been debated over the years, and initially it was thought to be related to problems in accurately measuring red cell mass and/or plasma volume. In particular, methods used for measuring the plasma volume may have led to an overestimation due to leaking of the marker into the interstitium. This appears to be a relatively minor problem if plasma volume measurement techniques are carried out with due attention to detail. It is now accepted that the F-cell ratio represents variations in the red cell mass distribution within the vascular space. The haematocrit in the microcirculation is lower than that in the macrocirculation. In essence this means that the red cell mass circulates faster than the plasma. The hae-modilution in small vessels was first postulated by Fahraeus and is now generally accepted. The individual organs of the body autoregulate their own microcirculatory flow and haematocrit by the process of vasomotion. The F-cell ratio has been shown to vary under different circumstances. Can the F-cell ratio of 0.9 be used as a constant when calculating the total blood volume by measuring only one of its components (i. e. red cell mass or plasma volume) or should both parameters be measured to ensure and accurate result? There has been controversy over the years as to whether the ratio is affected by anaemia or polycythaemia. Chaplin et al. originally demonstrated that there was consistency in the F-cell ratio over a wide haematocrit range [3]. As a result of these studies a standard correction factor has been used (usually F-cell ration of 0.9) when estimating red cell mass from plasma volume or vice versa. However, in recent years these findings have been disputed, and there is now agreement that there is a relationship between the haematocrit and the F-cell ratio and in the