Retrospective Analysis of Point-of-Care and Laboratory-Based Hemoglobin A 1c Testing

Transcription

1 Retrospective Analysis of Point-of-Care and Laboratory-Based Hemoglobin A 1c Testing Jennifer L. Clark 1 and Lokinendi V. Rao 1 * Background: Glycemic control is essential to diabetic management, and hemoglobin A 1c (Hb A 1c ) has long been used for this purpose. Though laboratory-based testing is standard, point-of-care (POC) systems provide rapid results in clinic, allowing more timely patient management. A negative bias with POC testing has been observed, and our aim is to further characterize these discrepancies at our institution. Methods: A medical record search identified patients who underwent laboratory-based and/or POC Hb A 1c testing (DCA Vantage ) at our medical center from July 2015 to April Patients who underwent both tests within 30 days were grouped by age, sex, and test interval (same day, <1 day, 15 days, or 30 days). Mean laboratory-based and POC values were compared using the paired t-test. Correlation statistics were determined using the Deming regression. Results: In total, data points were gathered from the database, comprising laboratory-based Hb A 1c tests and POC-based Hb A 1c tests. A total of unique patients were identified, of which 493 underwent both tests within 30 days. While DCA and laboratory-based testing was highly correlated, there was a mean negative bias of 0.18% with POC testing. Bias was greater for women [0.17% higher (95% CI, 0.063% 0.284%), P = 0.002] and children aged 0 13 years [0.52% higher (95% CI, 0.141% 0.891%), P = 0.007]. Conclusions: There is a consistent negative bias with POC testing, most pronounced in the female and pediatric populations. Further studies will determine what variables contribute to this discrepancy and how clinical management is modified. POC testing using the DCA Vantage should be interpreted cautiously. IMPACT STATEMENT The aim of our study was to evaluate the degree of negative bias in point-of-care (POC) hemoglobin A 1c (Hb A 1c ) testing, an observation that has been widely reported, in the diabetes clinic at our institution. On the basis of our data, we were able to identify specific groups within the diabetic population most at risk for discrepant testing that could adversely affect clinical management. Given these findings, clinicians may more critically evaluate POC Hb A 1c values, particularly in these vulnerable populations, before making major modifications to a patient's treatment plan. 1 Department of Pathology, UMass Memorial Medical Center, University of Massachusetts Medical School, Worcester, MA. *Address correspondence to this author at: Hospital Labs, Pathology, UMass Memorial Medical Center, One Biotech, 365 Plantation St., Worcester, MA Fax ; lokinendi.rao@umassmemorial.org. DOI: /jalm American Association for Clinical Chemistry 2 Nonstandard abbreviations: Hb A 1c, hemoglobin A 1c ; POC, point-of-care. March : JALM 1 Copyright 2017 by American Association for Clinical Chemistry.

2 Point-of-Care and Lab-Based Hb A 1c Testing Diabetes is the sixth leading cause of diseaserelated death in the United States, contributing to a wide variety of comorbidities including heart disease, kidney disease, and blindness (1). Glycemic control is central to the successful management of these patients (2). Hemoglobin A 1c (Hb A 1c ), 2 or glycohemoglobin, measurements have long been used as an indicator of glycemic control in diabetic patients, representing the mean blood glucose levels over the previous 120 days, the mean lifespan of the red cell (2 4). When increased, glycemic control may be enhanced through changes in diet, lifestyle, and medication regimens with repeat testing every 3 6 months to monitor progress (5). Laboratory-based Hb A 1c levels are generally preferred, with a goal <7.0% in most diabetic patients (5). However, test results are normally obtained after patients have already left their office visit. Point-of-care (POC) testing systems, which can provide a result while the patient is still in the office, have been designed to bridge that gap, providing real-time feedback to the patient and physician and allowing more efficient and effective management decisions (6 8). Despite the demonstrated benefits of POC testing in this setting, concerns remain regarding the accuracy of finger-stick Hb A 1c testing (9, 10). Though studies have shown good correlation overall between laboratorybased and POC measurements, a systematic negative bias has been frequently observed with the finger-stick testing (9, 11 13). The aim of the current study is to address observed bias in our own laboratory and identify particular groups most at risk for falsely decreased POC Hb A 1c levels. MATERIALS AND METHODS A retrospective medical record search was performed to identify all patients who underwent laboratory-based and/or POC Hb A 1c testing at our institution during a 10-month time period from July 2015 to April Those patients undergoing both tests within 30 days or less were identified. The presence or absence of a hemoglobinopathy was not known. Groups were compared based on test interval (same day, within 1 day, within 15 days, or within 30 days), age, and sex. To correlate the values of the 2 tests within these groups, correlation coefficient (R), slope, and y-intercept were calculated based on the Deming regression. To determine the statistical significance of the mean difference between laboratory-based and POC Hb A 1c values between groups, the P value was calculated using a Student t-test. Laboratory-based testing was performed using the Roche Tina-quant Gen.2 Hb A 1c turbidimetric inhibition immunoassay on the Integra 800 analyzer (Roche Diagnostics). Hb A 1c determination is based on turbidimetric inhibition immunoassay of hemolyzed whole blood samples. Hb A 1c in the sample reacts with anti Hb A 1c antibody, forming a soluble antigen-antibody complex. Polyhaptens react with excess anti Hb A 1c, and the formation of an insoluble antibody-polyhapten complex is turbidimetrically determined. In the central laboratory, 2 levels of QC material are run twice daily. The precision at level 1 (Hb A 1c concentration of 5.5%) QC material was <1%, and precision at level 2 (Hb A 1c concentration of 9.9%) was 0.7%, respectively. POC testing was performed using DCA Vantage (Siemens Medical Solutions Diagnostics, Tarrytown, NY), which is based on latex agglutination inhibition immunoassay methodology. The system consists of a spectrophotometric and precalibrated, unitized reagent cartridge containing both wet and dry reagents. The immunological reaction uses a monoclonal antibody, and light scattering is quantified from absorbance measured at 530 nm with simultaneous total hemoglobin evaluation using potassium ferricyanide. RESULTS A total of data points were gathered from the laboratory database, comprising JALM :05 March 2017

3 Point-of-Care and Lab-Based Hb A 1c Testing ARTICLE Table 1. Comparison of laboratory-based and POC Hb A 1c tests performed within 30 days. a Testing interval n Mean lab-based Range of lab-based Mean POC Range of POC R Slope y- Intercept P A Same day <0.001 b B 1 day <0.001 b C 15 days <0.001 b D 30 days <0.001 b a R, slope, and y-intercept were calculated by using the Deming regression. The P value was calculated by using the paired t-test for mean lab-based and POC-based Hb A 1c tests. A D indicate corresponding graph in Fig. 1. b Significance. laboratory-based Hb A 1c tests (70.5%) and POC Hb A 1c tests (29.5%). Of the laboratory-based tests, (85.3%) were performed on samples from outpatients, and approximately 30% of test values fell within normal limits. All POC testing was performed in the diabetes clinics, and the majority (93.8%) of tests yielded increased Hb A 1c levels. Patient sex was relatively equal in the dataset with 51% females and 49% males. Patient age ranged from 1 year, 11 months, to 105 years, with a median age of 57 years. Within the total dataset, unique patients were identified, and many of which had multiple tests during the study period. Those patients who underwent both laboratory-based and DCA testing within 30 days or less were identified, consisting of 499 instances. Six patients with both laboratorybased and DCA values >14% were eliminated, since this is the limit of the reportable range for the DCA test, leaving 493 in the study group. Within this group, 246 (49.9%) were female and 247 (50.1%) were male. Ages ranged from 2 years, 7 months, to 89 years, with a median age of 57 years. Only 21 (4.2%) patients in this group exhibited Hb A 1c levels within the normal range during the study period. Both tests were obtained on the same day in 145 (29.4%) instances, within 1 day in 245 (49.7%) instances, and within 15 days in 372 (75.5%) instances. The mean interval between tests was 8 days overall, with children aged 0 10 years demonstrating the longest mean test interval at 18 days. The mean laboratory-based and DCA values, ranges, correlation coefficient (R), slope of regression line, y-intercept, and P values are summarized in Table 1 for each test interval group. Correlation coefficients between laboratory-based and DCA values always exceeded 0.95 with slopes ranging from to and y-intercept ranging from 0.34 to Based on the paired t-test, there was a statistically significant difference between the laboratory-based and DCA values in each test interval group (P <0.001 for all groups). The mean laboratory-based and DCA values within Hb A 1c ranges are shown for each test interval group in Fig. 1. Mean laboratory-based values were higher than mean DCA values in the range of 6.0% 11.0% with outliers in the very low and very high ranges. Overall, DCA values were on average 0.18% (actual bias) lower than laboratory-based values. The actual bias for each patient and mean bias are shown for each test interval group in Fig. 2. The actual difference in reported test results varies up to 1 2 points (more toward negative side), and this bias is greater at higher Hb A 1c levels. This result could be attributable to poor peripheral circulation in the uncontrolled diabetic population with higher Hb A 1c levels. Most assays should fall within <0.3% bias according to the National Glycohemoglobin Standardization Project (NGSP) (14). In our study, 14.6% of cases showed an overestimate of Hb A 1c on DCA testing by 0.3%, and 41.8% of cases showed an underestimate of Hb A 1c on DCA testing by 0.3%. Overall, 56.4% of patients had discrepant DCA test March : JALM 3

4 Point-of-Care and Lab-Based Hb A 1c Testing Fig. 1. Mean laboratory-based and POC-based Hb A 1c values at different level intervals (range from 4.1% to 14%) for patients undergoing both tests. The overall mean test values are shown at each Hb A 1c range for same-day testing (A), testing interval 1 day (B), testing interval 15 days (C), and testing interval 30 days (D). Hb A 1c ranges are based on laboratory-based value. Error bars represent SE. Statistical significance is indicated as follows: *P <0.05, **P <0.01, ***P < results exceeding the maximum allowable error. These results are summarized in Table 2. In addition to identifying statistically significant test discrepancies, we determined the extent of clinically significant discrepancies in our data set using the generally accepted threshold of 0.5% points Hb A 1c (15). Overall, we found that the DCA value underestimated the Hb A 1c by 0.5% in 106 cases (21.5%). Of these, 63.2% were female. Interestingly, the DCA value overestimated the Hb A 1c by 0.5% in 42 cases, and 61.9% of these were male. Altogether, clinically significant discrepancies were found in 148 cases (30.0%). These results are summarized in Table 2. Mean laboratory-based Hb A 1c values were higher than DCA values in all age-groups (Fig. 3), and this result was statistically significant based on the paired t-test (0 13 years: P = 0.018, >13 years: P <0.001). The difference between tests was largest in children aged 0 13 years, who demonstrated higher laboratory-based values than DCA values by a mean of 0.69% (actual bias) compared to the >13 years of age population, who demonstrated laboratory-based values just 0.18% higher. The mean test discrepancy (laboratory-based value minus DCA value) between the 0 13 years of age and >13 years of age populations was statistically significant (P = 0.007) 4 JALM :05 March 2017

5 Point-of-Care and Lab-Based Hb A 1c Testing ARTICLE Fig. 2. Actual bias between laboratory-based and DCA Hb A 1c testing for each test interval group: same day testing (A), testing within 1 day (B), testing within 15 days (C), and testing within 30 days (D). based on a Student t-test. These results are summarized in Table 3. The 2 tests were also compared based on sex (Fig. 4). Females demonstrated mean laboratory-based values of 8.51% and mean DCA values of 8.24%, with a mean difference of 0.27%. Males demonstrated mean laboratory-based values of 8.36% and mean DCA values of 8.26%, with a mean Table 2. Number of laboratory-based Hb A 1c tests under- or overestimated by the POC test using 0.3% laboratory bias threshold and 0.5% clinically significant bias threshold out of 493 total cases. 0.3% Discrepancy 0.5% Discrepancy POC underestimated Hb A 1c 206 (41.8%) 106 (21.5%) POC overestimated Hb A 1c 72 (14.6%) 42 (8.5%) Total discrepant 278 (56.4%) 148 (30.0%) March : JALM 5

6 Point-of-Care and Lab-Based Hb A 1c Testing Fig. 4. Mean laboratory-based and DCA Hb A 1c values by sex. The mean test values are shown for females and males. Error bars represent SE. Statistical significance is indicated as follows: **P <0.01, ***P < between laboratory-based and DCA values within the male and female groups (females: P <0.001, males: P = 0.005) based on the paired t-test. These results are summarized in Table 3. Fig. 3. Mean laboratory-based and DCA Hb A 1c values by patient age. The mean test values (A) and mean difference between test values (B) are shown for patients age 0 13 years and >13 years. The mean difference was calculated by subtracting the DCA value from the laboratory-based value. Error bars represent SE. Statistical significance is indicated as follows: *P <0.05, **P <0.01, ***P < difference of 0.10%. The test discrepancy (laboratorybased value minus DCA value) between males and females was statistically significant (P = 0.002) based on a Student t-test, as was the difference DISCUSSION We show in the current study that there is a consistent negative bias with finger stick DCA testing when compared to laboratory-based testing. This discrepancy on average is 0.18% and is statistically significant. This discrepancy certainly carries clinical weight, since small changes in Hb A 1c levels, even those below the accepted 0.5% clinically threshold, may trigger modifications in clinical management (15). A number of patients in our Table 3. Summary of laboratory-based and POC Hb A 1c tests by sex and age. n Mean lab-based Range of lab-based Mean POC Range of POC P a 0 13 years b >13 years <0.001 b Male b Female <0.001 b a P value was calculated by using the paired t-test. b Significance. 6 JALM :05 March 2017

7 Point-of-Care and Lab-Based Hb A 1c Testing ARTICLE study actually demonstrated discrepancies in laboratory-based and DCA testing greatly exceeding 1% point, an unacceptable deviation from the generally accepted laboratory bias threshold <0.3% in the actual numbers reported. Overall, more than half of patients showed a discrepancy of 0.3% or greater between the 2 tests, and approximately 20% of patients included in the study showed a clinically significant discrepancy based on a threshold of 0.5% for clinical significance. The clinical ramifications of these discrepancies are not known and deserve consideration in further studies. We did identify 2 groups at risk for potentially clinically significant discrepancies on finger-stick Hb A 1c testing. Females demonstrated a greater discrepancy than males in this study with a mean negative bias of nearly 0.3% on DCA testing, which was statistically significant. Though this discrepancy is still small, less than the 0.5% generally considered to be clinically significant, this group was clearly more prone to test discrepancies and therefore had increased risk of inadequate glycemic management (16). Notably, the majority of patients whose DCA test underestimated the Hb A 1c by 0.5% or greater were female. The most striking result in this study was the highly clinically significant discrepancy seen in children aged 0 13 years. In this group, there was a mean negative bias approaching 0.7% on DCA testing. Such a discrepancy would certainly affect clinical management in this particularly vulnerable population. These results may indicate that the finger-stick technique used on women and children by some phlebotomists is not ideal for POC Hb A 1c testing or that there is some physiological difference in pediatric and female populations that affects Hb A 1c levels in the subcutaneous capillary beds. Further studies will be needed to answer this question. There are several limitations to this study, due in particular to its retrospective nature. Ideally, the study would be done prospectively with all patients undergoing both tests on the same day by the same phlebotomist with a standardized fingerstick technique. In addition, more young children should be enrolled in subsequent studies, as the pediatric population in the current study was quite limited. It is important to note that these data pertain to testing done at a single institution and may not represent diabetic populations in all locales. However, we believe our results may be generalized to most diabetic patients seeking care at large, urban tertiary care centers using the same testing methods. In conclusion, POC testing should be interpreted cautiously in the context of clinical decision-making, particularly in pediatric and female populations. In addition, care should be taken to use adequate finger-stick technique when performing POC tests in these groups. Author Contributions: All authors confirmed they have contributed to the intellectual content of this paper and have met the following 4 requirements: (a) significant contributions to the conception and design, acquisition of data, or analysis and interpretation of data; (b) drafting or revising the article for intellectual content; (c) final approval of the published article; and (d) agreement to be accountable for all aspects of the article thus ensuring that questions related to the accuracy or integrity of any part of the article are appropriately investigated and resolved. Authors Disclosures or Potential Conflicts of Interest: No authors declared any potential conflicts of interest. Role of Sponsor: No sponsor was declared. REFERENCES 1. National Center for Health Statistics. Health, United States, 2015: with special feature on racial and ethnic disparities. Washington (DC): U.S. Government Printing Office; 2016:449 p. 2. Gonen B, Rubenstein A, Rochman H, Tanega SP, Horwitz DL. Haemoglobin A1: an indicator of the metabolic control of diabetic patients. Lancet 1977;2: March : JALM 7

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