Abstract

Spaceflight has adverse effects on the human body that pose health risks to astronauts spending extended time in space missions. For clinical monitoring of astronauts and for in-flight biomedical research, laboratory instruments must be available in the spaceflight environment. Currently, no instrument has been shown to be capable of generating a white blood cell (WBC) count and differential during spaceflight to our knowledge, although this is a medical requirement of the National Aeronautics and Space Administration (NASA). We evaluated a compact hematology analyzer for compatibility with a zero-gravity environment. We performed analyses in reduced-gravity during parabolic flight. Herein, we describe our engineering evaluation and report the reduced-gravity validation data we collected. The hematology analyzer we tested met the basic requirements for use in spaceflight and should be capable of accurately measuring WBC parameters aboard the International Space Station.

Effects of Spaceflight on the Human Body

Conditions during spaceflight cause many adverse health effects in humans, including loss of bone density and muscle strength due to hypokenesis and altered cardiac function due to significant fluid redistribution. The risk of cancer also increases with prolonged radiation exposure. Recent evidence1–3 indicates that immune function is altered during spaceflight; causal factors may include crewmember stress, isolation, altered nutrition, and/or disrupted circadian rhythms. Also, microgravity may have a direct effect on T-cell activation due to altered trans-duction of intracellular signals.4Microgravity is defined as the near zero-gravity environment of low-Earth orbital spaceflight. While orbiting the Earth, gravity still exerts most of its downward force (surface, 9.8 m/s2; Earth orbit, 9.0 m/s2). However, beyond the influence of atmospheric drag, orbital flight balances the force of gravity on the spacecraft by an outward force created by the tremendous speed of the craft as it circles the Earth (∼27,359km/hr). This balance is defined as microgravity because it is not technically zero gravity (although objects still appear to float).

Crewmembers in spaceflight are also exposed to environmental dangers such as toxins that may accumulate in the closed environment and increased reactivation and shedding of latent herpes viruses.5,6 During Earth orbital flight, however, crews have remained in generally good health because countermeasures such as exercise, improved diet, and fluid intake offset bone and muscle loss to a certain extent. Immune system alterations during orbital flight do not usually result in clinical disease. However, the health risks associated with spaceflight generally increase as the mission length increases. One of the primary objectives of the International Space Station (ISS) is to investigate the medical consequences of prolonged spaceflight and develop and validate countermeasures necessary to enable deep-space missions beyond Earth orbit. A long-term goal of the United States space program is to develop heavy-lift rocket capabilities and crew vehicles that can enable exploration of near-Earth asteroids, the moon, and Mars.

Laboratory Instruments and Spaceflight

To enable medical monitoring of crewmembers and to perform basic clinical research, medical laboratory instruments are required aboard the ISS. However, very few laboratory instruments are currently deployed on the ISS because of technical constraints. The iSTAT Portable Clinical Analyzer (Abbott Laboratories, Abbott Park, IL) and a portable ultrasound imaging device have been flown successfully as scientific payloads. No other diagnostic medical instruments are currently available for use in spaceflight; most laboratory instruments are incompatible with spaceflight conditions. Most hematology analyzers and flow cytometers are large and heavy, have significant power requirements, use large amounts of reagents, and generate significant amounts of biohazardous waste. Instrument fluidics may be altered in microgravity; an example is the laminar flow/hydrodynamic focusing used in flow cytometers.7 These properties are potentially incompatible with spaceflight, although it should be possible to reengineer some laboratory instruments to achieve compatibility with spaceflight.8

Without clinical laboratory instruments, medical research onboard the ISS may be performed on specimens collected during flight but returned for terrestrial analysis. This pushes to capacity the storage space requirements aboard space vehicles returning to Earth, which have been extremely limited in the post-Shuttle era. Frozen samples can be collected at any time and stored on the space station before they return to earth for use. To return ambient, live blood samples, the samples can only be collected right before undock, and immediately returned. It is highly desirable to have instruments for in-flight medical and research requirements. To our knowledge a suitable instrument for measuring white blood cell (WBC) count or differential during spaceflight has not been identified. The ability to obtain a WBC count is an unmet National Aeronautics and Space Administration (NASA) medical requirement during space-flight9 because no available analyzer has been validated for use in the unique environment of spaceflight.

To be adaptable to spaceflight conditions, a laboratory instrument must be small, use minimal power, require little or no maintenance, use minimal liquid reagent, generate minimal liquid biohazardous waste, and have a mechanical design that is robust and vibration tolerant. It is desirable for reagents to be stable at room temperature for prolonged periods of time since refrigerated storage is severely limited. The user interface should be simple and intuitive, and analysis time must be rapid. Crew time in orbit is very limited; crews routinely perform several procedures at once. Most laboratory instruments do not meet these requirements.

Prototype In-Flight WBC Analyzer

Recently, a novel WBC/differential analyzer (WBC-DIFF) was developed by Hemocue, Inc. (Brea, CA, Image 1). This device is small (∼6.0 × 6.0 ×7.5 inches), can operate on battery power, and does not use liquid reagents. Instead, a 10-μl blood sample is aspirated into a cuvette containing a lysing reagent and a nuclear staining dye (Image 2). The cuvette is inserted into the instrument, and a charge-coupled device (CCD) camera captures an image of the specimen, producing a WBC and differential via nuclear morphology. The analysis takes approximately 3 minutes and 20 seconds. NASA occasionally uses commercially available equipment to support in-flight requirements, sometimes with no or minimal modifications. In most cases, commercially developed instruments have been extensively validated as a prerequisite for clinical use. Although designed for use in small laboratories and physicians’ offices, we identified the WBC-DIFF analyzer as being potentially compatible with spaceflight because it met most of the requirements previously described.

According to the technical specifications available from Hemocue, Inc, the instrument works within a range of 0.3 to 30.0 ×109 WBC/L; counts above or below this range generate an error message. The clinical validation of this instrument was described by Osei-Bimpong et al,10 who found that its results did not vary by greater than 5.0% versus an automated hematology analyzer (Beckman Coulter Inc, LH-750, Brea, CA). Linearity within the detection range was verified. The accuracy of the WBC-DIFF was validated using 500 blood specimens spanning the detection range; less than 10% deviation from the reference method was observed and no bias was evident at the low or high ends of the line arrange. The authors noted that artificially elevated WBC counts were observed in certain clinical conditions (eg, sickle-cell disease and thalassaemia major); the WBC-DIFF did not alert the user to these results.10

In 2012, we obtained a NASA Human Health and Performance Directorate innovation grant to the Immunology Laboratory of the Johnson Space Center, Houston, TX, to evaluate the WBC-DIFF for use in spaceflight. In this study, we purchased and evaluated a WBC-DIFF instrument, performed cursory validation to confirm the performance of the instrument, and undertook an engineering evaluation under spaceflight conditions, followed by a reduced-gravity evaluation aboard a NASA parabolic-flight aircraft.

Image 1

White blood cell and differential blood cell analyzer by Hemocue, Inc. (Brea, CA). A, Exterior views. B, Interior views.

Image 1

White blood cell and differential blood cell analyzer by Hemocue, Inc. (Brea, CA). A, Exterior views. B, Interior views.

Image 2

Sampling cuvette for the Hemocue, Inc. (Brea, CA) white blood cell (WBC-DIFF) analyzer. A, The cuvette is designed to aspirate a finger-stick sample. B, After collection, a lysing reagent lyses the red-blood-cell population within the optical scanning area. C, The cuvette also contains a DNA dye that stains the WBC nuclei (original magnification, ×20). We used nuclear morphology to resolve the various WBC subsets.

Image 2

Sampling cuvette for the Hemocue, Inc. (Brea, CA) white blood cell (WBC-DIFF) analyzer. A, The cuvette is designed to aspirate a finger-stick sample. B, After collection, a lysing reagent lyses the red-blood-cell population within the optical scanning area. C, The cuvette also contains a DNA dye that stains the WBC nuclei (original magnification, ×20). We used nuclear morphology to resolve the various WBC subsets.

Materials and Methods

Samples

Venous blood samples from healthy subjects were collected in ethylenediaminetetraacetic acid (EDTA) anticoagulant tubes and stored at room temperature until analysis. For the engineering evaluation, 8 test subjects participated in the confirmatory terrestrial comparative studies and 2 other subjects participated in the reduced-gravity evaluation. Parallel finger-stick blood samples were collected via lancet into WBC/differential microcuvettes (Hemocue, Inc.) according to the manufacturer’s instructions. Except where indicated (ie, in a sample-stability time-course study), all finger-stick samples were analyzed within 10 minutes of collection.

Instruments

For the reference method, we performed analysis of venous blood samples in a College of American Pathologists (CAP)–certified clinical laboratory with a Coulter LH-750 hematology analyzer (Beckman Coulter Inc, Brea, CA). Test data were obtained by analyzing venous or finger-stick samples on the WBC-DIFF. The WBC-DIFF instrument generates an absolute WBC count and calculates percentages of neutrophils, lymphocytes, monocytes, eosinophils, and basophils.

Comparison Studies

Statistical analyses were performed by Stata IC software (version 12.1, StataCorp LP, College Station, TX) using the 2-tailed alpha and a threshold of statistical significance of P <0.05. We used the concordance correlation coefficient (ρ) reported by Lin11,12 to evaluate agreement among measurements of WBC, granulocytes, lymphocytes, eosinophils, basophils, and monocytes by the reference hematology analyzer and the WBC-DIFF device. We also evaluated agreement between analyses of venous blood and finger-stick specimens. We calculated the mean differences between measures taken with the WBC-DIFF device versus those taken using the reference analyzer, and between venous and finger-stick specimens; we report the 95% limits of agreement of these differences.13

Results

Laboratory Validation

Although extensive validation data for the WBC-DIFF is available from its manufacturer, we performed basic validation studies in our laboratory on the delivered unit. In venous blood samples, we found the results generated by the WBC-DIFF to be reasonably comparable to the reference hematology analyzer results for all measured parameters except monocytes (Figure 1). The statistical measures of concordance (SE) were as follows: WBC, .90(.07); granulocytes, .91(.07); lymphocytes, .87(.07); eosinophils, .94(.04); basophils, .60(.16); and monocytes, −.27(.18). Average differences (SD) between the reference and WBC-DIFF measurements were WBC: .11(.38); granulocytes: .25(3.20); lymphocytes: −3.63(1.06); eosinophils: .21(.70); basophils: .11(.30); and monocytes: 2.66(2.97). Using only the WBC-DIFF analyzer, we evaluated the concordance between venous and finger-stick blood specimens. The results of these studies are summarized in Table 1. The findings agree with a published report14 that demonstrated finger-stick blood is an acceptable sample type for complete blood cell (CBC) measurements. We evaluated the stability of specimens in filled cuvettes over time; this may also be an indicator of intra-cuvette precision. Using a finger-stick sample, the filled cuvettes were stable for approximately 30 minutes. Errors due to evaporation of specimen from the filled cuvettes became significant after longer periods. Cuvette stability data are presented in Image 3.

Engineering Evaluation

We performed an engineering evaluation of the WBC-DIFF for compatibility with requirements for flight hardware and the possibility of engineering improvements to size or power consumption. The instrument has minimal unused internal space (Image 1). On loading a filled cuvette onto the plastic holder, the holder is manually moved along a guide into the view of the CCD camera. During movement, 2 magnets hold the sample in place and then immobilize the sample in the view of the camera; we found this to be an optimal sample-handling method for reduced-gravity operations. For zero-gravity evaluations, we determined that only minor modifications to affix the cuvette within the holder were required, which we accomplished by simply adding adhesive material to the cuvette-holding area. We determined that no other engineering changes were required for our reduced-gravity evaluation.

Figure 1

Concordance correlation coefficient (ρ) evaluation of agreement of blood-outcome measures between the WBC-DIFF (x-axis) and standard methodology (y-axis). White blood cell levels are expressed as 103 cells/mL; relative percentage data are plotted for all other measures. The standard instrument for comparison was the Coulter LH-750 hematology analyzer (Beckman Coulter Inc, Brea, CA).

Figure 1

Concordance correlation coefficient (ρ) evaluation of agreement of blood-outcome measures between the WBC-DIFF (x-axis) and standard methodology (y-axis). White blood cell levels are expressed as 103 cells/mL; relative percentage data are plotted for all other measures. The standard instrument for comparison was the Coulter LH-750 hematology analyzer (Beckman Coulter Inc, Brea, CA).

Table 1

Concordance Correlation Coefficient Evaluation (ρ) to Determine Agreement of Blood-Outcome Measures Between Venous and Fingerstick Measurements Using Only the WBC-DIFFa

Image 3

Data indicating stained and lysed blood samples remain stable within the WBC-DIFF reagent cuvette for approximately 30 minutes. Analysis was performed via the WBC-DIFF instrument. A, Representative subject data from analysis immediately after collection (0:00) to analysis at +45 minutes after collection. Note that the instrument was able to generate a white blood cell (WBC) count after the loss of differential resolution. Stability varies, and it is suggested that analysis take place immediately following sample collection. The primary reason for loss of sample integrity is desiccation of the filled cuvettes (B).

WBC, white blood cell; neut, neutrophils; lymph, lymphocytes; mono, monocytes; eos, eosiniphils; basos, basophils.

Image 3

Data indicating stained and lysed blood samples remain stable within the WBC-DIFF reagent cuvette for approximately 30 minutes. Analysis was performed via the WBC-DIFF instrument. A, Representative subject data from analysis immediately after collection (0:00) to analysis at +45 minutes after collection. Note that the instrument was able to generate a white blood cell (WBC) count after the loss of differential resolution. Stability varies, and it is suggested that analysis take place immediately following sample collection. The primary reason for loss of sample integrity is desiccation of the filled cuvettes (B).

WBC, white blood cell; neut, neutrophils; lymph, lymphocytes; mono, monocytes; eos, eosiniphils; basos, basophils.

Evaluation of hardware in reduced gravity is possible in a parabolic flight aircraft (Image 4). A NASA jet flies in a parabolic arc; a 30-second zero-gravity period is produced at the peak of the parabola, whereas a 2G force (twice the normal gravitational force) is generated at the bottom of the parabola. Many parabolic paths are flown in a repeated sequence to facilitate zero-gravity testing. In this evaluation, we successfully collected a series of finger-stick blood specimens into reagent cuvettes during the zero-gravity phase of the flight (Image 5). To determine whether the analyzer was sensitive to gravitational effects, we performed analyses during parabolic flight. However, because the instrument requires more than 3 minutes to generate a result, it was not possible to restrict analysis to within the zero-gravity phase of flight. Therefore, we initiated the analytical cycle at the beginning of a parabolic path and allowed the analysis to continue to completion (Image 4). The analysis took place during a series of 3 consecutive zero-to-2G phases, with no intervening 1-fold (ie, normal) gravity-contamination condition. We conclude that a successful operation, spanning zero-gravity and 2G phases of flight, indicates that the instrument (which was continually analyzing images during the 3-minute analysis period) is capable of functioning properly in either gravity condition. Data from 2 separate flights, with 1 test subject evaluated per flight, were collected. Control data at normal gravity was generated during level aircraft flight. Our data indicated that the WBC-DIFF functioned properly in normal, zero-, or 2-fold gravity conditions (Figure 2).

Image 4

Zero-gravity evaluation of the WBC-DIFF, which took place aboard a National Aeronautics and Space Administration (NASA) parabolic flight aircraft. A, Aircraft. B, Coauthor performing analysis. This aircraft generates approximately 30 seconds of zero gravity by flying in a parabolic arc. Because analysis takes longer than 30 seconds to complete, analysis spanned repeated zero-gravity and 2-fold gravity parabolas C, Graph showing the effects of hypogravity and hypergravity, which were determined simultaneously.

Image 4

Zero-gravity evaluation of the WBC-DIFF, which took place aboard a National Aeronautics and Space Administration (NASA) parabolic flight aircraft. A, Aircraft. B, Coauthor performing analysis. This aircraft generates approximately 30 seconds of zero gravity by flying in a parabolic arc. Because analysis takes longer than 30 seconds to complete, analysis spanned repeated zero-gravity and 2-fold gravity parabolas C, Graph showing the effects of hypogravity and hypergravity, which were determined simultaneously.

Image 5

Finger-stick blood sample collection during zero-gravity phase of parabolic flight.

Image 5

Finger-stick blood sample collection during zero-gravity phase of parabolic flight.

Figure 2

Representative single-subject data from the WBC-DIFF. The instrument yielded acceptable data when analysis spanned the zero-gravity and 2-fold gravity conditions (in flight), compared with the normal-gravity control data. Two separate analyses are shown for flight conditions, 1 of which covered the normal-gravity control condition.

WBCs, white blood cells; grans, granulocytes; lym, lymphocytes; mono, monoctyles; eos, eosiniphils; basos, basophils.

Figure 2

Representative single-subject data from the WBC-DIFF. The instrument yielded acceptable data when analysis spanned the zero-gravity and 2-fold gravity conditions (in flight), compared with the normal-gravity control data. Two separate analyses are shown for flight conditions, 1 of which covered the normal-gravity control condition.

WBCs, white blood cells; grans, granulocytes; lym, lymphocytes; mono, monoctyles; eos, eosiniphils; basos, basophils.

Study Limitations

This study was the first evaluation of a hematology instrument for use aboard the ISS. The evaluation was necessarily limited because of the unique conditions under which the instrument would have to operate. Validation of a new instrument or method would be comprehensive, conforming to CAP or Clinical Laboratory Improvement Amendments (CLIA) standards. Typical validation studies involve hundreds of parallel measurements on both the candidate and the reference instrument, along with studies to establish a normal range, confirm the sensitivity and linearity of measurements, and assess potential interferences. A benefit of using commercial technology (in this case, an FDA-approved method) is that extensive validation studies have already been performed by the manufacturer. However, these validation data do not entirely replace in-laboratory validation of a new instrument in the environment where the instrument will be operated. The WBC-DIFF instrument was assessed for use aboard the ISS. It would not be feasible to deploy the instrument to the ISS and perform the full spectrum of validation studies to comply with CAP/CLIA requirements. The primary goal for this study was to conduct engineering and microgravity evaluations of this instrument to determine its suitability for use in spaceflight. We considered the instrument size, power consumption, vibration tolerance, reagent stability, and reliable function in microgravity. In this report, we cited existing validation data and performed a basic check of the analyzer. Limitations also exist regarding the availability and cost of the parabolic-flight microgravity evaluation opportunities: only 1 aircraft exists in the United States that supports this flight profile. Each flight consists of 30 parabolas, each containing approximately 25 seconds of microgravity time. Only 2 flights were available for this study, which was a severe limiting factor. Therefore, we developed an optimization strategy that allowed photographic and video recording of finger-stick sample collection during the microgravity phase for multiple operators. We collected microgravity and terrestrial hardware evaluation data for a single test subject on each flight. Although a normal terrestrial instrument evaluation would evaluate a significantly greater number of test subjects, we were able to achieve a successful microgravity evaluation within these constraints.

Discussion

The recently developed WBC-DIFF hematology analyzer is miniaturized, robust, and has an appropriate amount of reagent storage to satisfy the requirements of space-flight (eg, microgravity compatibility, size, minimal power consumption). This instrument is available for clinical use in Europe; however United States Food and Drug Administration (FDA) approval is currently pending. In this study, a limited validation assessment versus a standard hematology instrument yielded mostly acceptable correlations; however, monocyte percentages measured by the evaluation unit were significantly different from measurements made on the reference analyzer (Figure 1). Studies are currently underway aboard the ISS to characterize space-flight-associated immune dysfunction. Currently it is not known whether specific cellular populations are altered in peripheral blood or have altered functional characteristics. Although it may be important to monitor all WBC subpopulations, use of the WBC-DIFF for measuring the WBC and certain differential parameters has value. Given the current absence of in-flight hematology capabilities, the parameters validated in this instrument would augment current capabilities. Before selection of the WBC-DIFF for spaceflight, we anticipate that FDA approval and a full validation of the differential capabilities of the instrument would be required.

Using the WBC-DIFF analyzer only, finger-stick samples produced results generally similar to those from venous blood samples (Table 1), in agreement with literature previously published study.14 As validated in the current study, the WBC-DIFF instrument is able to perform analyses in a zero-gravity environment (Image 2). For all parameters except monocyte percentages, we consider this instrument to be acceptable for use during spaceflight.

The primary goal of this study was to determine whether this WBC-DIFF instrument is capable of functioning in reduced gravity. NASA requires further engineering evaluations before actual deployment to the space vehicles, possibly including tests of radiation tolerance, electromagnetic emissions, release of any toxic compounds into the environment, and sensitivity to vibrations. Vibration tolerance is a particular concern in the launch phase of deployment. Modifications to any candidate devices may be implemented to improve performance in any of these areas of concern. For example, vulnerable components such as lenses and mirrors may be additionally secured, circuit boards may be coated, and heat transfer may be improved by ventilation or fans. The results of our evaluation indicate that the WBC-DIFF instrument is suitable for performing WBC counts and certain differential components aboard the ISS. The instrument is small, uses minimal power, and delivers rapid results. Reagents (ie, very small cuvettes) are stored in desiccated packaging and do not require refrigeration. Therefore, the reagents should be stable during the time frame required for space missions, and we would anticipate them to be relatively insensitive to increased radiation levels. Deployment of this analyzer would address a currently unmet need in orbit and enhance crew health and safety. It is appropriate to use the ISS as a test platform for development of this technology before its use on deep-space missions. Currently, the Human Health Countermeasures Element of NASA’s Human Research Program is investigating the requirements to flight certify the WBC-DIFF analyzer for a demonstration aboard the ISS.

The authors thank the NASA-Johnson Space Center (JSC) Internal Research and Development (IR&D) Program for funding this study, the JSC Clinical Laboratory for providing control data, and the JSC Reduced Gravity Office for facilitating the parabolic-flight opportunity.

To read this article online, scan the QR code, http://labmed.ascpjournals.org/content/44/4/304.full.pdf+html

Abbreviations

  • ISS

    International Space Station;

  • WBC

    white blood cell;

  • NASA

    National Aeronautics and Space Administration;

  • CCD

    charge-coupled device;

  • EDTA

    ethylenediaminetetraacetic acid;

  • CBC

    complete blood cell;

  • CLIA

    Clinical Laboratory Improvement Amendments;

  • IR&D

    Internal Research and Development

References

1
Williams
D
Kuipers
A
Mukai
C
Thirsk
R
.
Acclimation during space flight: effects on human physiology
.
Can Med Assoc J
 .
2009
;
180
(
13
):
1317
1323
.
2
Gueguinou
N
Huin-Schohn
C
Bascove
M
et al
Could spaceflight-associated immune system weakening preclude the expansion of human presence beyond Earth’s orbit?
J Leukoc Biol
 .
2009
;
86
(
5
):
1027
1038
.
3
Crucian
B
Sams
C
.
Immune system dysregulation during spaceflight: clinical risk for exploration-class missions
.
J Leukoc Biol
 .
2009
;
86
(
5
):
1017
1018
.
4
Boonyaratanakornkit
JB
Cogoli
A
Li
CF
et al
Key gravity-sensitive signaling pathways drive T cell activation
.
FASEB J
 .
2005
;
19
(
14
):
2020
2022
.
5
Wilson
JW
Ott
CM
Höner zu Bentrup
K
et al
Space flight alters bacterial gene expression and virulence and reveals a role for global regulator Hfq
.
Proc Natl Acad Sci U S A
 .
2007
;
104
(
41
):
16299
16304
.
6
Cohrs
RJ
Mehta
SK
Schmid
DS
Gilden
DH
Pierson
DL
.
Asymptomatic reactivation and shed of infectious varicella zoster virus in astronauts
.
J Med Virol
 .
2008
;
80
(
6
):
1116
1122
.
7
Crucian
B
Norman
J
Brentz
J
Pietrzyk
R
Sams
C
.
Laboratory outreach: student assessment of flow cytometer fluidics in zero-gravity
.
Lab Med
 .
2000
:
31
(
10
):
569
573
.
8
Crucian
B
Guess
T
Quiriarte
H
Sams
C
.
Flow Cytometry in Space: To help mitigate risk to astronauts, spaceflight-compatible clinical laboratory instruments must be developed
. ADVANCE Healthcare Network for Laboratory Web site. http://laboratory-manager.advanceweb.com/Features/Articles/Flow-Cytometry-in-Space-Part-1.aspx. Accessed September 3, 2013.
9
NASA Space Flight Human System Standard–Volume 1: Crew Health (NASA-STD-3000 and JSC 26882)
.
Space Flight Health Requirements Document
 .
2009
;Vol.
1
, Chap. 7.
10
Osei-Bimpong
A
Jury
C
McLean
R
Lewis
SM
.
Point-of-care method for total white cell count: an evaluation of the HemoCue WBC device
.
Int J Lab Hematol
 .
2009
;
31
(
6
):
657
664
.
11
Lin
LI-K
.
A concordance correlation coefficient to evaluate reproducibility
.
Biometrics
 .
1989
;
45
:
255
268
.
12
Lin
L
.
A note on the concordance correlation coefficient
.
Biometrics
 .
2000
;
56
:
324
325
.
13
Bland
JM
Altman
DG
.
Statistical methods for assessing agreement between two methods of clinical measurement
.
Lancet
 .
1986
;
1
:
307
310
.
14
Rao
LV
Moiles
D
Snyder
M
.
Finger-stick complete blood counts: Comparison between venous and capillary blood
.
Point of Care
 .
2011
;
10
:
120
122
.