Lab Quality Results at the Point of Care
About A1CNow Accuracy and Performance
A1CNow+ Fingerstick Comparative Testing
(NGSP-certified method is the Tosoh A1C 2.2 Plus)
A1CNow+ Venous Comparative Testing
(NGSP-Certified method is the Tosoh A1C 2.2 Plus)
Venous blood was collected from 110 diabetic subjects, and each sample was tested on one of three different lots. Aliquots of the venous samples were also tested by the NGSP-certified method, providing comparative results. Data analysis again consisted of least squares linear regression (x = reference results), bias calculation, and Bland-Altman limits. The data are provided below.
- Professional product insert P/N 90821B.
Precision is the quality of being repeatable in amount or performance. Precision of a test is observed when measurement of a sample has nearly the same value each time it is measured. Precision testing was done under a specialized protocol. Following this protocol, two whole blood samples, one of approximately 6.0% A1C (low), and one of approximately 9.0% A1C (high), were tested over 20 days and four runs per day, for a total of 80 assays per level. The overall imprecision (including within-day and between-day) was 3.00% CV at the low level and 4.02% CV at the high level.1
- Professional product insert P/N 90821B.
The NGSP standardizes glycated hemoglobin test results so that clinical laboratory results are comparable to those reported in the Diabetes Control and Complications Trial (DCCT) where relationships to mean blood glucose and risk for vascular complications have been established. A key component of the program is the Reference Laboratory Network. The network interacts with manufacturers of glycohemoglobin methods to assist them first in standardizing their methods and then in providing comparison data for certification of traceability to the DCCT.
The NGSP certification process includes the exchange of 40 patient blood samples and a method comparison analysis. A certificate of traceability to the Diabetes Control and Complications Trial (DCCT) Reference Method is issued to any manufacturer or laboratory that successfully completes the certification process. The certificate is effective for 1 year and is specific to the reagents and instrumentation used during certification. The process consists of a 40 sample comparison with a Secondary Reference Laboratory (SRL). In order for a commercial method to be considered traceable to the Central Primary Reference Laboratory, the 95% CI of the differences between methods (test method and SRL method) must fall within the limits of ±0.75% A1C. Effective 9/1/2012, the NGSP certification requirements have been significantly tightened and 37 of 40 results must be within +/- 6% (relative) of the reference (true) value when measured in the 4-10% A1C range. The new NGSP criteria applies to methods annually certified after 9/1/2012, and will thus apply to all certified methods by 9/1/2013. Changes in NGSP criteria generally require a 1 year roll over period for currently certified methods.
A1CNow test systems are IFCC-traceable. The relationship between the National Glycohemoglobin Standardization Program (NGSP) A1C calibration (expressed as % A1C) and the International Federation of Clinical Chemistry (IFCC) network calibration (expressed as mmol/mol) has been shown to be stable and is given by the master equation: (NGSP = [0.09148 * IFCC] + 2.152). For further details and additional references, see the NGSP website at http://www.ngsp.org/.Since A1CNow test systems are NGSP-certified results can be converted to IFCC units with the master equation.
Demonstrating IFCC traceability is a formal process during which A1C blood samples are assigned values by the IFCC network. Manufacturers use these samples to verify alignment of their assay with the IFCC network.
National Glycohemoglobin Standardization Program webpage, http://www.ngsp.org
There will always be differences between multiple test results due to normal variation, time between tests, and differences between methods. In fact, duplicate test results from the same patient will rarely match even when the test is performed using the same method at the same time. Differences can sometimes appear greater because one result could be slightly higher and the other could be slightly lower than the reference (true) value. Positive and negative biases are often present in all of the most frequently used A1C testing methods. Multiple A1C methods should not be used to evaluate a
single patient or the performance of an A1C testing device because large and sometimes unacceptable inter-method differences are possible when multiple A1C testing devices are used for an individual patient.1
1 Holmes, E.W. et. al. (2008). Analytic Bias among Certified Methods for the Measurement of Hemoglobin A1c. American Journal of Clinical Pathology, 129, 540-547.
While in most people Hemoglobin A accounts for 95 to 98% of their hemoglobin, some individuals have genetic variations (such as Hemoglobin S or Hemoglobin F) that could affect the accuracy of a HbA1c test.
For more information, please refer to Sickle Cell Trait and Other Hemoglobinopathies and Diabetes: Important Information for Physicians (National Diabetes Information Clearinghouse)