Haemoglobinopathies are one of the most common inherited diseases worldwide. Quantification of haemoglobin A2 is necessary for the diagnosis of the beta thalassaemia trait. In this context, it is important to have a reliable reference interval for haemoglobin A2 and a local reference range for South Africa has not been established.
This study aimed to establish reference intervals for haemoglobin A2 using stored patient laboratory data.
This descriptive study used retrospective data to evaluate haemoglobin A2 levels determined using high-performance liquid chromatography at the National Health Laboratory Service haematology laboratory in Pretoria, South Africa. All tests performed from 01 October 2012 to 31 December 2020 were screened for inclusion; of these, 144 patients’ data met the selection criteria. The reference interval was calculated using descriptive statistics (mean and standard deviation) with a 95% confidence interval.
Analysed data from enrolled patients showed a normal distribution. The mean age of the patients was 40 years (range: 3–84 years). The reference interval for haemoglobin A2 calculated from this data was 2.3% – 3.6%. The minimum haemoglobin A2 was 2.3% and the maximum was 3.9% with a mean of 2.95% and a standard deviation of 0.357%.
A normal reference interval has been established for the population served by the laboratory that will assist with accurate diagnosis of the beta thalassaemia trait. This reference interval may also be useful to other laboratories that employ the same technology, especially smaller laboratories where obtaining a sufficiently large number of normal controls may be challenging.
Reference intervals are an integral part of any functioning clinical laboratory. Reference intervals are often the endpoint used by clinicians in the management of patients and in clinical decision-making. It has been estimated that 80% of medical decisions are made from laboratory results.
An example of where reference intervals are of diagnostic utility is in the diagnosis of the beta thalassaemia trait. The increase in haemoglobin A2 (HbA2) level is the most important laboratory parameter for the identification of carriers of beta thalassaemia, and is considered diagnostic in the appropriate clinical context.
Beta thalassaemia is inherited in an autosomal recessive pattern and the beta thalassaemia trait is estimated to have a prevalence of 1.5% worldwide, affecting approximately 80–90 million people.
Reference intervals are established through a validation process with a statistically adequate number, ideally 120, of healthy reference individuals.
Every laboratory that performs HbA2 testing is responsible for establishing its own reference interval. This is done by quantifying the HbA2 percentage in a cohort of healthy adults who do not have iron deficiency or the thalassaemia trait.
In keeping with good laboratory practice, a need was identified to determine HbA2 reference intervals in a local African population for the Department of Haematology of the National Health Laboratory Service (NHLS), Tshwane Academic Division (TAD), Pretoria, South Africa. Using a sufficient number of results from medical records would provide a healthy cohort for establishing a reference interval without the need to recruit healthy individuals. This is particularly useful for a test that is not routinely requested and only offered by specialised laboratories. In order to establish a reference interval for HbA2 without the limitations inherent to traditional methods of establishing reference intervals, we made use of previously reported normal high-performance liquid chromatography (HPLC) results from the NHLS Department of Haematology.
Approval was obtained from the Academic Affairs, Research and Quality Assurance Department of the NHLS. The study protocol was approved by the Faculty of Health Sciences Research Ethics Committee of the University of Pretoria (protocol number 518/2019). Patient consent was waived by the ethics committee as the study was conducted using historical data. Participants’ information was treated with confidentiality. Each participant was allocated a unique study number to ensure anonymity.
This was a descriptive study using retrospective data from blood test results stored in the Laboratory Information System of the NHLS at the Department of Haematology, TAD, and Vermaak and Partners Path Care Pathology group.
The study population comprised patients investigated by TAD, NHLS, and included patients from both high-income and low-income settings and represented a variety of ethnicities. All HPLC results of tests performed at the NHLS TAD from 01 October 2012 to 31 December 2020 were screened for inclusion (
Flow diagram of the study design, Pretoria, South Africa, 01 October 2012 – 31 December 2020.
Apart from HPLC reports, corresponding results of CBC, thyroid stimulating hormone, serum folate, serum vitamin B12 and ferritin level, performed within a week of the taking of the HPLC specimen, were evaluated. Only normal HPLC results were included in the analysis.
The HbA2 levels were determined in the haematology laboratory using the HPLC D10 instrument (Bio-Rad® Laboratories, Hercules, California, United States). The D10 instrument uses ion-exchange HPLC technology to analyse haemoglobin. All analyses were performed according to good laboratory practice and the manufacturer’s recommendations. The haematology laboratory at the NHLS TAD has been accredited by the South African National Accreditation System. Appropriate controls and calibrators were used throughout the study. The Lyphochek® Hemoglobin A2 Control Level 1 and 2 (Bio-Rad Laboratories, Irvine, California, United States) were used as controls and the D10TM Dual Program HbA2/F/A1C Calibrator/Diluent Set (Bio-Rad® Laboratories, Hercules, California, United States) was used for calibration.
The blood test results listed above were captured in a Microsoft Excel (Microsoft Corporation, Redmond, Washington, United States) spreadsheet. Exclusion criteria were applied after which 144 samples with normal HbA2 levels were identified, and these were used for calculation of reference intervals.
The descriptive statistics mean, median, standard deviation and inter-quartile range, with 95% confidence interval, and the 2.5th and 97.5th percentiles were used to describe the continuous variables such as HbA2 levels. The two-sample
The mean age of the 144 patients included in this data analysis was 40 years (range 3–84 years). The study population comprised 67 female and 77 male patients. After a single outlier of 4.3% was excluded from the analysis, the mean HbA2 value was 2.95% with the range between 2.2% and 3.9%. The standard deviation was 0.357% (
Box-plot of haemoglobin A2 distribution, Pretoria, South Africa, 01 October 2012 – 31 December 2020.
Data were normally distributed as indicated in the Kernel density estimation (
Kernel density estimate demonstrating the normal distribution of data, Pretoria, South Africa, 01 October 2012 – 31 December 2020.
A sex comparison was performed by
Scatterplot of haemoglobin A2 and age distribution, Pretoria, South Africa, 01 October 2012 – 31 December 2020.
The HbA2 reference interval established from this data set was 2.3% – 3.6%. Normal reference intervals for HbA2 have been published for other patient populations. A study performed at the Leiden University in the Netherlands, using the Variant Classic HPLC (Bio-Rad®) platform, reported a HbA2 reference interval of 2.3% – 3.5%.
Most evidence suggests that HbA2 of > 4% is indicative of beta thalassaemia trait with almost 100% sensitivity and 90% specificity.
The upper limit of the normal reference interval established in this current study does fall within the previously described ‘grey zone’. However, in the presence of a normal CBC, the possibility of an underlying carrier state in these patients remains small. This highlights the importance of interpreting HbA2 within the clinical context, taking into consideration the CBC parameters and iron studies.
There appears to be a weak association between a decreasing HbA2 value and increasing age; this was true even when looking at subsets of age and when excluding patients aged 70 years or older. Although this trend was seen, when calculating the reference interval by age, the reference interval remained 2.3–3.6 when rounded to one decimal place. Therefore, this was not a significant finding.
In this current study we used data available on the laboratory information system in order to establish a reference interval. This represents a novel approach in our setting. Data mining is emerging as an alternative to the traditional direct a priori method. Data mining makes use of electronic data records and statistical techniques to determine the healthy population within a data set in order to establish a reference interval.
The analytical methods that have been employed to establish reference intervals include The Hoffmann method, Bhatacharya method and more recently the Truncated Maximum Likelihood method.
One of the limitations of this study was the inability to exclude all confounding factors. Although our study could be improved on by only including patients who have been tested for all confounding factors for HbA2, this would only be feasible if a large number of data sets were included. Another option would be to use a larger cohort and an algorithm that does not require the exclusion of all confounders. A normal CBC was used as a surrogate marker of a nutritional deficiency, and this remains a limitation. It should be noted that all published confounders are not routinely considered when establishing reference intervals for HbA2 or when interpreting HPLC results.
A normal HbA2 reference interval of 2.3% – 3.6% has been established for the population served by the laboratory. This will assist with the interpretation of results. The reference interval could also be useful to other laboratories, especially smaller laboratories where obtaining a sufficiently large number of normal controls may be challenging. The inability to exclude all of the confounding factors that influence HbA2 levels needs further research.
We thank Prof. R. Pool for his insight and guidance and Dr D. Toi for his assistance in proofreading the article.
The authors declare that they have no financial or personal relationships that may have inappropriately influenced them in writing this article.
J.P. and T.N. conceived of the presented idea. C.N. developed the theory and performed the computations. J.P., C.N. and T.N. verified the analytical methods. J.P. supervised the findings of this work. All authors discussed the results and contributed to the final manuscript.
Funding has been provided by the University of Pretoria Development Fund.
The data that support the findings of this study are available on request from the corresponding author, C.N.
The views expressed in the submitted article are the authors’ own and not an official position of the institution.