A Comparative Study of Blood Viscometers of 3 Different Types

The greater the viscosity of the blood is, the more di�cult the �ow of theblood becomes, and the incidence of diseases caused by blood circulation disorders increases. Diseases related to increased viscosity are commonly associated with the cardiovascular and cerebrovascular system. [1,2] High blood viscosity is the cause of circulatory system diseases. Studies showing that blood viscosity can be accurately measured and applied in clinical trials to prevent diseases of the circulatory system can be found in the literature. [3] Viscosity data can exhibit variations depending on the viscosity measurement methods, even if the methods are rooted in hydrodynamic principles. Even though it is an approved blood viscometer, the results of blood viscosity often differ depending on the type of viscometer. This has the potential to create confusion within the medical �eld. Informing whether measurement results differ depending on the viscometer and what the level of error is for each measurement method will help reduce confusion in the medical community. To our knowledge, the degree of difference in viscosity measurement results due to differences in these measurement methods and the cause of the difference have not yet been explored. In this study, three blood viscosity measurement methods registered with the Ministry of Food and Drug Safety of Korea were selected to study the same canine blood. The viscosity measurements were carried out using each device and compared. The parallel plate and scanning capillary methods had similar viscosity values, while the cone plate method had lower viscosity values. The viscosity of blood, as measured by the three viscometers, differed, and more experimental data must be accumulated to evaluate the cause of the difference between these methods. In this paper, we pointed out several causes of inconsistency and suggested some measures for people to avoid this confusion. However, con�rming that the test results show systematic differences is expected to help clinicians who diagnose and prescribe patients using blood viscosity results. The results of this comparative study are expected to be the starting point for the establishment of guidelines or standards for blood measurement methods.


Introduction
Viscosity is the physical quantity that represents the value at which the uid resists ow and indicates how tight the uid is.The viscosity of the blood is the most direct hemodynamic factor that determines the ow characteristics of the blood in blood vessels.
Blood is a nonhomogenous uid in which round, at, and exible solid masses are lled with very high concentrations when expressed via rheology.These solid masses tend to clump together under low shear rates and disperse at high shear rates.Measuring the viscosity of blood with these characteristics is more complicated than measuring the viscosity of homogenous substances.
Recently, clinical results revealing that high blood viscosity is closely related to diseases of the circulatory system, especially ischemic stroke, have drawn increased amounts of attention from the medical community [2][3][4][5][6].In another study, blood viscosity increased in patients with acute ischemic stroke [4], and it was con rmed that blood viscosity was associated with neurological deterioration in ischemic stroke patients.[7,8,9] Blood clots lead to an increase in blood viscosity.The high viscosity of blood increases the shear stress between the blood vessel and the blood, leading to stroke.[3,4] As an increasing number of people use blood viscosity as a basis for determining a patient's condition because blood viscosity affects brain and heart diseases, various types of blood viscosity systems are registered with the Ministry of Food and Drug Safety of Korea (alternatively called the KFDA).
All these methods have been developed based on hydrodynamic theories, but the results may differ because they measure viscosity under different ow conditions.
The clinical reality is that users are confused because they do not know what to believe or how to interpret the difference when they see different viscosity measurements when the test method is different.Despite this confusion in the eld, there are no data comparing the data measured by each method, weakening the willingness of medical personnel to derive actionable conclusions that can lead to better care of patients from blood viscosity information.
There are three methods for analyzing blood viscosity that are currently approved by the KFDA for use as in vitro diagnostic medical devices.The instruments used were a parallel plate viscometer (PP), a scanning capillary viscometer (Capillary), and a cone-plate viscometer (CP).The fourth method, relative viscosity measurement, was recently approved by the KFDA in April 2021.[10] This exploratory study seeks to identify the differences between these approved methods, identify the cause, and improve the reliability of blood viscosity measurements in the medical eld.

Methods
This study focused on evaluating three blood viscosity systems that are o cially registered with the KFDA.The chosen samples were taken from canine blood, providing a convenient and accessible source.
To ensure proper testing conditions, blood coagulation was prevented using a CPDA-1-coated blood bag during collection.Viscosity assessments were performed on whole blood without separating its components.Before viscosity was measured, the blood was gently agitated at room temperature to prevent sedimentation, and viscosity readings were taken at 37°C.All viscosity measurements were completed within three days of blood collection.
The following is a concise overview of the three blood viscosity measurement methods and the respective equipment employed in this experiment: PP: This instrument gauges the viscosity by interposing the uid between two facing plates.One plate rotates sinusoidally clockwise and counterclockwise, and the torque is measured.It assesses both viscosity and elasticity.The ARS-Medi viscometer used here has previously been used in biorelated elds, including blood viscosity tests [11,12].Its accuracy was veri ed against a precision engineering viscometer [13].The diameter of the plates was 60 mm, and the gap between the two plates was 0.5 mm.The strain or amplitude of oscillatory motion was 100%.Unlike the two transient methods described below, this method employs a dynamic test mode.
Scanning: Viscosity determination involves measuring the uid's velocity as it traverses a slender Ushaped tube.High-viscosity uids take longer to pass through the capillary tube because of their slower speed, whereas low-viscosity uids pass rapidly.The relative viscosity was calculated as the ratio of the passage time through the capillary tube.Hemovister, the device used in this study, has applications in stroke research [13].
CP: This viscometer determines viscosity by rotating a spindle immersed in uid and assessing the resistance to rotation.After the spindle is spun at high speed, the speed of rotation and the torque exerted until the spindle stops due to blood are measured to calculate the viscosity.An SA-5600 device was used for this method.
Figure 1 shows pictures of the viscometers used in this study and illustrates the basic motions of each machine and the fundamental principles of the blood viscosity systems.The details pertaining to measurements involving each piece of equipment, such as samples, sample quantities, and reporting units, are summarized in Table 1.
Upon sample collection, the sample is introduced into a blood viscometer, where the viscosity is measured according to the distinctive procedure of each viscometer.The resulting viscosity values are represented in units of cP (centipoise) or mPas (millipascal sec).

Results
Viscosity measurements were performed consistently at 37°C (± 0.2°C) for all three types of viscometers.The data from 10 experiments for each measurement method are shown in Tables 2~4 and Figure 2.
Table 2. Canine blood viscosity (cP) was measured by the parallel plate method.
Table 3. Canine blood viscosity (cP) was measured by a scanning capillary method.
Table 4. Canine blood viscosity (cP) was measured by the cone-plate method.
Page 7/12 All three types of visual devices exhibit shear thinning behavior in canine blood, as shown in Figure 2 (a), (b), and (c).Shear thinning in this case signi es a decrease in blood viscosity as the shear rate (/s) or angular frequency (rad/s) increases.This observed trend aligns with a widely recognized pattern in blood rheology, as indicated by previous researchers.The alteration in the overall uid ow resistance arises from the interaction between the clumping and dispersion of red blood cells, coupled with their deformation.
Different patterns emerge in the graphs that depict viscosity measurements at different shear rates, as shown in Fig. 2 (d), (e), and (f).The results at a shear rate of 5/s (5 rad/s) are presented in Fig. 2 (d); both the parallel plate method and the scanning capillary method exhibit almost identical viscosities, while the cone plate method displays a viscosity that is half that of the preceding two methods.A notable aspect of this gure is the signi cantly smaller experimental deviation associated with the CPT.
Similarly, Fig. 2 (e), which compares viscosity at a shear rate of 10/s (10 rad/s), follows the same trend as (d), and Fig. 2 (f), which represents viscosity at a shear rate of 30/s (30 rad/s), does not deviate from this pattern.However, a slight variance emerges between the parallel plate method and the scanning capillary method.
Comparison of viscosity values is predominantly conducted under low shear rates (5, 10, and 30/s or 5, 10, and 30 rad/s).This decision stems from the acknowledgment, as mentioned by Song et al. [14], that blood ow at high shear rates is dominated by inertia forces rather than viscous forces.Moreover, disease occurrence due to blood ow obstruction primarily manifests in regions characterized by low velocity and narrow blood vessels, where shear rates are naturally low.Consequently, viscosity, which is pertinent to clinical outcomes, is associated with low shear rates.
In Fig. 2, the viscosity measured using the parallel plate method closely approximates the result obtained with the capillary method.In contrast, the viscosity measured by the cone plate method is approximately half that of the other two methods.

Discussion
The expectation when viscosity is measured using the same sample is that different measurement methods should yield the same or similar values.However, it has been established through direct experimentation that viscosity comparisons exhibit variations depending on the chosen measurement approach.These disparities are believed to arise from various biophysical factors, including the in uence of the surface tension of blood and the effects of secondary ow within regions with high shear rates.Another factor is that the CP and capillary parameters are used to measure the viscosity of blood not in the steady state but in the transient state.
It is very di cult to determine the root cause of these differences, but several causes can be inferred from the guidelines for the viscosity of human blood published earlier.According to Bull et al., whole blood should be well mixed before the measurement starts.Since platelets in the blood aggregate, they have a greater viscosity than nondispersed platelets.For a capillary viscometer, mixing before measurement is not an easy task.
The capillary and cone plate methods measure viscosity in transient states that are not normal.Transient states sometimes cause signi cant errors, especially for materials with memory effects.Other guidelines [16] even exclude blood viscosity measurements in 'scanning' mode or transient states.
The viscosity measured by a cone plate instrument is very low compared to that measured by parallel plates and capillary methods.The reason for this difference can be assumed to be that the cup of the cone plate instrument rotates at a high speed (~ 3000 rpm), and the rotational speed decreases as the measurement progresses.At high speeds, the clumped blood cells are dispersed and do not regroup until the end of the test.This means that high shear rates affect the low shear rate viscosity measurements.
The parallel plate method measures viscosity not in a steady state but in a dynamic state.The PP adapts to large amplitude oscillatory shear (LAOS).PP avoids the high-speed effect that can be seen in the CP method and the mixed effect that can be seen in the capillary method.
In essence, blood viscosity is quite low, demanding the use of highly precise detection equipment, but the current state of available medical technology presents limitations in this regard.
To date, there has been no reference material or reference method for measuring blood viscosity in its entirety.Until a benchmark is established, the derivation of correlations between measurement methods becomes essential to mitigate clinical confusion and produce consistent, harmonized outcomes.
The pivotal factors in clinical viscosity measurements are the ease of measurement and the stability of the acquired values.In particular, when viscosity measurements are employed to forecast the onset of cardiocerebral blood-related disorders or future disease progression, it becomes imperative to generate stable information regardless of the medical institution or examiner under the assumption of maintaining a certain level of quality control.Consequently, factors such as the reproducibility of the results under identical patient conditions, the complexity of the testing method, and the potential impact of the sample storage duration must be considered when determining a standardized viscosity measurement methodology.
This study is signi cant because it comparatively analyzed three different commercial blood viscosity systems.Given the increasing interest in blood viscosity within the cardiovascular domain, the prompt establishment of a standard capable of serving as a benchmark for future medical research methodologies has become a pressing priority.
To summarize, the measurements were dependent on the test method, with the parallel plate and scanning capillary methods showing similar viscosity values varying between 5 and 20 cps, while the cone plate method showed lower viscosity values between 3 and 10 cps.These disparities may be due to biophysical factors such as the surface tension of blood, secondary ow effects, and differences in conditions between transient and steady-state measurements.In this context, it is essential to establish a reference method for blood viscosity after standardizing the biophysical conditions underlying the differences between measurement methods as an approach, such as the case of the erythrocyte sedimentation rate (ESR) measured by a biophysical blood test [17].
Guidelines made before commercial blood viscosity measurements become widely available need to be changed to suit healthcare industry requirements.The potential guidelines for whole blood viscosity should include premixing blood and maintaining a steady state for simple shear ow and maintaining an angular frequency for dynamic shear during measurements.

Declarations
Figures

Figure 1 Three
Figure 1

Table 1
Comparison of three types of blood viscometer