Decades of investigations were focused on finding “gold standard” for evaluation of plasma dilution and osmolality, blood loss evaluation and prediction of bleeding or transfusion induced changes in hematocrit and hemoglobin concentration. Addressing deficiencies of existing methods, the current invention created new combined mathematical-physiological model applicable to manually operated nomograms and software in medical monitors. The mathematical model HBS Trends is used in blood transfusion and infusion therapy nomogram—HBS Nomogram—which is based on blood hemoglobin concentration and hematocrit. It is also an easy and practical tool for recording and dynamical interpretation of plasma osmolality, blood hemoglobin concentration, hematocrit and mean corpuscular hemoglobin concentration. The HBS Nomogram is a practical system for organizing blood test results in a patient's medical records. It can be used alone or, in line with existing guidelines for infusion and transfusion therapy making them more practical, cost effective and time saving in decision making.