Technical Bulletin, Pronto-7 Accuracy - Masimo

Technical Bulletin
optical interference. Two consecutive noninvasive total hemoglobin (SpHb) readings were obtained from each subject,
detaching and reattaching the sensor between each measurement. Perfusion index, finger temperature, and SpO2,
displayed by the noninvasive hemoglobin test device, were also recorded. Testing was conducted while patients were still,
quiet and sitting upright with their hands on the tabletop.
Immediately following the noninvasive testing, a venous blood sample was obtained by venipuncture of the median
cubital vein of the non-dominant arm (when obtainable) with a disposable syringe and then transferred to 2-mL vacuum
tube containing EDTA (Ethylene-DiamineTetraacetic Acid), an anticoagulant. Venous blood samples were transported
at room temperature and analyzed for reference hemoglobin (tHb) with an automated hematology analyzer (Coulter ®
LH 500, Beckman Coulter, USA 6 ) per Clinical and Laboratory Standards Institute guidelines 8 and the manufacturer’s
directions for use, within 24 hours of collection. The laboratory analyzer was calibrated daily per the manufacturer’s
recommendations and good laboratory practice. The laboratory hematology analyzer used in this study measures
hemoglobin by lysing red blood cells then converting the hemoglobin to a stable pigment. The mass of the hemoglobin
in the sample is then determined by the degree of absorbance of the pigment. The accuracy of this method has been
validated compared to the hemoglobincyanide method, the reference method for hemoglobinometry recommended by
the International Committee for Standardization in Hematology (ICSH). 9
A capillary blood sample was obtained by finger-stick with a 2.25 finger lancet and analyzed immediately with a point
of care hemoglobinometer (HemoCue ® 201+, HemoCue a division of Quest Diagnostics, Cypress, CA 10 ) according to
the manufacturer’s directions for use. The HemoCue is a photometric device that uses microcuvettes that contain
three reagents to lyse red blood cells then convert hemoglobin into methemoglobinazide. The absorbance of
methemoglobinazide is then measured to estimate total hemoglobin. 11 The precision of HemoCue for the measurement
of hemoglobin varies depending on the type of sample. HemoCue is reported to overestimate hemoglobin from capillary
samples by 10-15% compared to the hemoglobincyanide method. 12
Accuracy of SpHb and total hemoglobin by the point of care device (POC-tHb) was assessed by calculating bias and
standard deviation of each test method compared to laboratory tHb. To assess repeatability of SpHb measurement,
precision (coefficient of variation) between the first and second SpHb measurements was calculated. Precision was
calculated as the standard deviation of the difference between the two SpHb test results divided by the mean SpHb value
multiplied by 100 and expressed as a percent.
Lastly, multivariate analysis was conducted to evaluate the associations of patient characteristics to SpHb-tHb bias. Variables
analyzed were body mass index, age, finger diameter, perfusion index, SpO2, finger temperature, ethnicity, skin pigment,
smoking status, comorbidities (high blood pressure, diabetes, high cholesterol, respiratory disorders, osteoarthritis) and
medications. Multiple logistic regression was performed with significant predictors from the univariate analyses (p≥0.05) to
determine main effects and interactions in a multivariate model. For retention in the model a cut off of p≤0.01 was used.
Results
A total of 474 subjects were enrolled. 34 subjects were excluded for the following reasons: 31 venous samples were not
properly transported and unable to obtain 3 noninvasive SpHb measurements. Of the remaining 440 subjects, 62% were male,
had an average age (±SD) of 36 (±18) years. Four hundred thirty subjects (98%) were from outpatient clinics and 10 (2%) were
from a community health screening event. Subjects consisted of 257 (58%) healthy volunteers and 183 (42%) outpatients
presenting with one or more conditions. The number and percent of the outpatients affected with the most common
conditions were 72 (16%) with diabetes, 68 (15%) with high cholesterol, 32 (7%) with high blood pressure, 21 (5%) with asthma,
17 (4%) with osteoarthritis, 13 (3%) with chronic obstructive pulmonary disease, and 3 (1%) with pregnancy. Patients had one or
more conditions. A total of 369 (84%) of all subjects had light pigmentation, 64 (15%) had medium pigmentation, and 7 (2%)
had dark pigmentation. Patient demographics, medical conditions, and medications are shown in Table 4.
Univariate and Multivariate Analysis of Factors Associated with SpHb Bias
Variables with both univariate and multivariate significance included Massey scale score and SpO2. For each increase in
Massey scale, SpHb bias increased by 0.16 g/dL. Higher biases were observed in darker pigmented patients (Massey scale
score of 4-9) but the number of subjects in this group was small. For each decrease in SpO2 value, SpHb bias decreased by
0.14 g/dL. Subjects with SpO2 of 96% or below therefore had a negative bias.