BACKGROUND AND OBJECTIVES: There is a need for simple method allowing detection of dehydration and hypovolemia. Based on a new theory of homeostatic blood states, we hypothesized that hemodilution following standardized crystalloid fluid bolus can be used to discriminate between baseline normohydration and dehydration, also normovolemia and hypovolemia. METHODS: Computer simulations based on previously published kinetic data were used to define the best time points for discrimination between baseline normohydration and dehydration, also normovolemia and hypovolemia. Hemodilution was compared at the proposed timing in 20 volunteers who received 40 infusions of Ringer’s solution of 25 mL/kg during 30 minutes. RESULTS: Simulations indicated that preexisting hypovolemia could be best detected at the end of infusion, while dehydration 20-30 min later. In baseline hypovolemia, the peak reduction of hemoglobin concentration was 16.0% at the end of infusion, while it was only 11.8%, when participants were normovolemic (P<0.004). In baseline dehydration, the residual hemodilution was 8.6%, when measured 30 min after the end of infusion. It was only 3.1% in baseline normohydration (P<0.006). CONCLUSIONS: In response to fluid load, the baseline dehydration exaggerates the lowering of residual hemoglobin in respect to baseline. Meanwhile, baseline hypovolemia exaggerates the lowering of peak hemoglobin concentration. The volume loading test that deploys interpretation of hemoglobin dynamics in response to the test volume load could possibly serve as an easily available guide to indicate an individual patient's baseline hydration state and volemia. The introduction of continuous noninvasive monitoring of hemoglobin concentration would expand the applicability of the new method. PMID: 19142053 [PubMed - indexed for MEDLINE