for the purposes of promoting an understanding of the principles of the invention , reference will be made to the embodiments illustrated in the drawings , and specific language will be used to describe the same . it will nevertheless be understood that these embodiments constitute no limitation as to the scope of the invention , with alterations and further modifications being permissible , and such further applications of the principles of the technology as illustrated therein , being contemplated as would normally occur to one skilled in the art to which the technology relates . described herein are methods for determining , adjusting and maintaining plasma levels in a patient receiving a blood volume expander as part of a therapeutic program or treatment regime . also described herein are improved blood volume expanders for directly measuring the amount of expander in a patient undergoing treatment . also described herein are uses of the improved blood volume expanders for the manufacture of a medicament for measuring , adjusting and maintaining blood volume in a patient . a fluorescently - labeled dextran is incorporated in a dextran - based plasma volume expander ( i . e . a small known amount of fluorescently - labeled dextran is mixed into a known amount of non - fluorescent dextran ) to permit the direct measurement of the amount of expander by measuring the intensity of the signal generated by the fluorescent marker in the volume expander . accordingly , and as may be used herein , the term “ biological marker ” or “ biomarker ” is intended to denote a molecular entity designed to be introduced into the vascular system of an animal , preferably a human subject , primarily to measure the function of an organ or vasculature of the animal . it is a biocompatible molecule formed as the conjugation product of one or more fluorescent molecules or dyes and a macromolecule . the terms “ volume expander ”, “ blood volume expander ”, and “ plasma volume expander ” are used synonymously herein and designate biocompatible compositions designed to restore vascular volume , stabilize circulatory haemodynamics , and maintain tissue perfusion . typical blood volume expanders include saline , albumin and dextran , such as dextran 40 , dextran 70 and dextran 75 , where the numerical designations refer to the molecular weight of the dextran . an “ unmodified ” or “ standard ” blood volume expander is a blood volume expander typically given to a patient in a clinical setting , and which does not include a labeled fluorescent component or molecule , or any other type of label or marker as a detection mechanism . the “ blood volume ” or “ plasma volume ” as used herein denotes the amount of plasma volume within the vascular space including arterial , venous and capillary spaces . the blood volume does not include the volume contributed by the blood cells , such as the red blood cells . the total blood volume is the blood volume and the volume contributed by the blood cells , which can be determined from the hematocrit or the packed cell volume . a “ non - metabolized ” molecule , or a molecule which is “ not metabolized within the subject ”, as used herein , is a molecule which is not significantly metabolized during the time in which the measurements are performed . such molecules typically have a half life of 4 hours or greater in the vascular system of the subject . by “ permeable to vessel walls ” is meant a molecule that can cross vessel walls . similarly , “ impermeable to vessel walls ” means that the molecule cannot cross the vessel walls either through a passive process or an active process . a “ dynamic molecule ” is a molecule of sufficiently low molecular mass to permeate the blood vessel walls or the vasculature of a subject . these molecules are also cleared by the kidney via glomerular filtration with a decrease in clearance as molecular weight increases . dynamic molecules are known in the art to have a molecular mass of less than about 75 kda . a “ static molecule ” is a molecule of sufficiently high molecular mass to significantly limit its blood vessel wall permeability and metabolism . static molecules or markers may reach a quasi - stable vascular concentration for a period of time , although such markers may ultimately be cleared from the vasculature . static markers are known in the art to have a molecular mass greater than about 75 kda , preferably about 150 kda . such markers can remain in the vasculature for a time period of between about 1 or 2 hours , up to 12 hours or longer , depending on the molecular mass of the marker and rate of metabolism of the marker . the term “ fluorescently - labeled crystalloid ” as used herein refers to adding a free , unconjugated fluorescent dye such as fitc into the solution . according to the disclosure , the blood volume of a patient can be conveniently measured , adjusted and maintained using the techniques and compositions described herein . an initial assessment of the patient &# 39 ; s blood volume is conducted , and once the initial assessment is completed , any loss in blood can be supplemented by administering a blood volume expander to the patient . blood volume can be measured directly by utilizing a static fluorescent molecule introduced into the vascular system of the subject by techniques such as disclosed , for instance , in published pct application pct / us2013 / 026277 , filed feb . 15 , 2013 , the disclosure of which is incorporated herein in its entirety by reference . this procedure , in general , involves the administration of a static marker suitable for the measurement of initial plasma levels in a subject . the static marker can be a fluorescently - labeled dextran having a molecular weight greater than about 75 kda to about 500 kda , and preferably about 150 kda . the fluorescent moiety used to label the dextran can be any of a variety of fluorescent molecules having a fluorescent excitation wavelength and an emission wavelength . the fluorescent moiety is conjugated with the dextran in a conjugation reaction by covalent attachment . the fluorescent molecule can be a fluorescent dye marker such as a rhodamine dye . suitable fluorescent molecules include , by way of example , texas red , fluorescein isothiocyanate ( fitc ) and 2 - sulfurhodamine ( 2shr ), with 2shr being preferred . the static marker is introduced into the subject by , for instance , intravenous injection , and a blood sample can be taken after about 20 to 30 minutes once the volume expander has fully distributed . the sample can be taken using an indwelling venous catheter or a measurement can be made after about 20 to 30 minutes using a non - invasive sensor , such as a non - invasive optical sensor . the blood sample can be analyzed to establish the blood plasma level as described in pct application no . pct / us2013 / 026277 . the use of fluorescently - labeled molecules to evaluate the physical condition of a subject by introducing the molecule into the vascular system of a subject has been disclosed . suitable methods and materials can be found in u . s . patent application ser . no . 12 / 425 , 827 , filed apr . 17 , 2009 and u . s . patent application ser . no . 12 / 946 , 471 , filed nov . 15 , 2010 , the respective disclosures of which are incorporated herein in their entirety by reference thereto . in general , according to the foregoing methods , a fluorescently - labeled macromolecule is introduced into the vascular system of a subject , such as by intravenous injection . once equilibrium has been reached , blood samples can be periodically withdrawn , every 10 to 20 minutes for example , and the samples are analyzed for fluorescence in a medium formed from the sample , a buffering solution and an anionic surfactant . the analysis is conducted using a light source to activate the fluorescent molecule , and a light detector is used to measure and quantify the intensity of the fluorescent signal generated by the activated molecule . the biometric parameter under evaluation ( blood volume ) can be efficiently determined by comparing the values obtained from the samples of the subject with samples from a known source . the blood volume expander can be any of a variety of blood volume expanders typically used by a clinician . suitable blood volume expanders include , for example , saline , albumin and dextran . in a typical clinical environment , the blood volume expander is administered to a subject in order to adjust and maintain the subject &# 39 ; s blood volume at a predetermined level . loss of blood typically occurs as a result of the treatment of the patient for a medical condition wherein blood loss is a significant factor or result . these conditions include severe trauma due to external injuries , and chronic renal disease and / or failure . the preferred blood volume expander is an unmodified or standard dextran solution , such as dextran 40 , dextran 70 or dextran 75 . the blood volume expander is “ doped ” or supplemented with a relatively small amount of a fluorescently - labeled dextran , typically from about 0 . 01 % to about 10 . 0 % by weight , preferably from about 0 . 01 % to about 1 . 0 % by weight , more preferably about 0 . 1 % by weight . the blood volume expander itself is typically an aqueous solution of dextran on the order of 5 % or 6 % of dextran by weight . the fluorescently labeled dextran can advantageously have about the same , near identical or identical molecular weight as the unmodified dextran . as noted , a dynamic molecule has a sufficiently low molecular weight to penetrate the blood vessel walls or vasculature of the patient . therefore , the amount of the molecule in the vasculature will change over time , and this change can be recorded , measured , and correlated with a biological parameter of interest , such as the level of blood volume . the fluorescently - labeled dextran can be prepared by conjugating a fluorescent molecule with dextran using reaction techniques and conditions well known in the art . typical fluorescent molecules which are suitable for conjugation include texas red , fluorescein isothiocyanate ( fitc ), and 2 - sulfhydrorhodamine ( 2shr ), with fluorescein being preferred . the “ doped ” marker dextran in the expander must have a different fluorescent marker than the “ static ” 150 kda dextran , which is preferably labeled with 2shr . according to the disclosure , a blood sample can be taken from a patient on a continuous or periodic basis , and the level of the volume expander in the blood can be measured by recording and correlating the presence of the dynamic molecule in the blood sample with the blood volume . this technique can be used to provide an accurate real time measurement of blood volume . the methods and compositions described herein can be typically used in a clinic or hospital where the treatment of renal disease and renal failure are indicated . the invention is further illustrated by the examples provided below , which are directed to certain embodiments of the invention and are not intended to limit the full scope of the invention as set forth in the appended claims . a 13 . 6 mg dose of a 2shr labeled dextran ( 150 kda ) is administered to a subject , and a blood sample is withdrawn after about 20 to 30 minutes to determine blood plasma volume using the procedure described in pct application no . pct / us2013 / 026277 . this blood plasma level is recorded and used as a reference level for additional blood plasma volume measurements . next , a standard dose of a dextran 40 or dextran 70 blood volume expander is administered to the subject with a small amount of a fluorescently - tagged dextran of approximately the same molecular weight added to the blood volume expander . for purposes of the example , a 30 gram solution of dextran 40 or dextran 70 is formulated with about 30 mg of approximately the same molecular weight dextran conjugated to fluorescein ( fitc ) ( or about 0 . 1 % of fluorescently - labeled dextran by weight of unmodified dextran ). blood plasma volume and fluorescent concentration ( ug / ml ) are recorded as a function of time for dextran 40 and dextran 70 . the results are shown in fig1 and 3 . fig4 shows that when using the static 2shr 150 kda marker to measure plasma volume , an increase in plasma volume caused by the volume expanders results in a decrease in the fluorescence of the static marker as it becomes more diluted . the compositions , uses thereof and methods described herein allow for the rapid determination of blood plasma levels . this facilitates the adjustment of patient fluid levels , such as the amount of blood plasma expander is required by the patient . this , in turn , can prevent renal damage due to a decrease in tubular flow as a result of renal blockage and stasis ( the cessation of urinary excretion ) which require adequate hydration as a treatment modality . while the invention has been illustrated and described in detail in the drawings and foregoing description , this is to be considered as illustrative and not restrictive in character . it is understood that one of ordinary skill in the art could readily make changes and modifications to the above - described embodiments and that it would be impractical to attempt to describe all such embodiment variations in the present specification . accordingly , it is understood that all such changes and modifications that come within the spirit of the invention are desired to be protected .