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 Table of Contents  
Year : 2019  |  Volume : 13  |  Issue : 3  |  Page : 102-105

Reference interval of fasting plasma glucose in apparently healthy adults in Port Harcourt, Nigeria

Department of Chemical Pathology, University of Port Harcourt, Port Harcourt, Rivers State, Nigeria, India

Date of Submission12-Sep-2019
Date of Acceptance12-Feb-2019
Date of Web Publication6-Jan-2020

Correspondence Address:
Frederick Igila Allison
Department of Chemical Pathology, Faculty of Basic Health Sciences, University of Port-Harcourt, Port Harcourt, Rivers State
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/phmj.phmj_20_19

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Background: Reference values of most assays in use in many chemical pathology laboratories in Nigeria are usually those of the manufacturer of diagnostic kit using non-indigenous reference populations. This is professionally unacceptable, so this study was aimed at determining a reference value for fasting plasma glucose (FPG) in adults living in Port Harcourt, Nigeria, using our indigenous population.
Methods: A total of 332 non-diabetic subjects who met the inclusion criteria for this study were recruited after they had signed the consent forms. After an overnight fast, about 2 ml of blood was collected into the fluoride oxalate bottles and was assayed the same day using the glucose oxidase method. Each sample was measured in duplicate and an average was calculated. The data were statistically analysed using the SPSS version 20 and the significance level was set at P ≤ 0.05.
Results: The reference interval of FPG was calculated by the non-parametric method after data had been screened for outliers and ranked. This was done using the 2.5th and 97.5th percentile due to the skewed nature of the data, and the reference interval of FPG obtained was 3.1–5.8 mmol/l.
Conclusion: The reference interval obtained from this study was different but close to that obtained in 2008 in Port Harcourt and more importantly different from those provided by manufacturers of the diagnostic kit (4.2–6.4 mmol/l). The use of this new reference interval is recommended to be necessary for better management of patients in Port Harcourt, Nigeria. The need to use locally determined reference intervals is emphasised.

Keywords: Diabetes mellitus, glucose oxidase method, reference interval

How to cite this article:
Allison FI, Ojule AC. Reference interval of fasting plasma glucose in apparently healthy adults in Port Harcourt, Nigeria. Port Harcourt Med J 2019;13:102-5

How to cite this URL:
Allison FI, Ojule AC. Reference interval of fasting plasma glucose in apparently healthy adults in Port Harcourt, Nigeria. Port Harcourt Med J [serial online] 2019 [cited 2023 Nov 30];13:102-5. Available from: https://www.phmj.org/text.asp?2019/13/3/102/275081

  Introduction Top

In most cities in Nigeria, the lifestyle of individuals is fast becoming sedentary and it is accompanied by an increase in the intake of 'fast food'.[1] This, therefore, has led to an increase in the incidence of obesity and metabolic syndrome and the conditions are associated with an increased incidence of type-2 diabetes mellitus, which has a prevalence rate of 4.6% in Nigeria [2] and 6.8% in Port Harcourt.[3] Since the incidence of diabetes in Port Harcourt is one of the highest in the country, it is important that reference intervals given by the manufacturers of diagnostic kits need to be verified for better patient management. It is important to have a population-based reference interval and those in existence are expected to be reviewed periodically [4] since population demographics are dynamic.[5] A local population-based reference interval will therefore improve the result interpretation and the overall management of diabetes patients. Most times, the upper cut-off limits of reference intervals pose a challenge as it determines the cut-off values for making diagnosis, treatment and monitoring of patients.

The aim of this study, therefore, was to determine the reference interval of fasting plasma glucose (FPG) among apparently healthy individuals living in Port Harcourt, the capital of Rivers State, Nigeria, and to compare the reference interval so generated with existing ones, both locally and internationally. The association between FPG and age and between gender and body mass index (BMI) was also evaluated.

  Methods Top

The study population consists of a cosmopolitan population of people living in Port Harcourt.

Approval of the Ethical Committee of the University of Port Harcourt Teaching Hospital was obtained before the commencement of this study. Subjects were selected by cross-sectional study method. A total of 332 apparently healthy subjects between the ages of 18 and 70 years who were willing to participate and signed the consent form were selected based on Reed's hypothesis.[6]

For those who could not read, the information was read to them and those who were willing to participate gave their consent by thumb printing the consent forms. Only those who gave their consent were recruited for the study. Information was obtained from the subjects with the help of a questionnaire. Subjects were told to eat their last meal of the day before 10 pm the previous night. Venous blood sample (2 ml) was taken between 7 and 10 am the next day into a fluoride oxalate bottle. This was gently rocked to allow the anticoagulant in the bottle mix with the blood sample. This sample was kept in a rack at room temperature and later centrifuged at 3000 rpm for 5 min to separate the plasma from the blood cells. This plasma was transferred into a plain sample bottle and batch analysed for FPG using the glucose oxidase method kit manufactured by Randox laboratories.[7]


One millilitre of glucose reagent was taken into the test tubes and 10 ml of blank, standard and blood samples are added to each of the tubes. This was gently mixed and left to stand for 10 min at room temperature and colour read at 520 nm using an Apel PD303S spectrophotometer, after zeroing the spectrophotometer with the blank. The concentration of plasma glucose was calculated as absorption of test over absorption of standard multiplied by the concentration of standard. This was done for all the samples, and each blood sample was measured in duplicate and the average value was taken to reduce errors.

The data generated were analysed using Statistical Package for Social Sciences version 20.0 (SPSS 20.0, IBM, USA). Outliers were eliminated using the Dixon outlier statistic [7] and the non-parametric method was used to determine the reference interval since the data were skewed and did not assume any particular distribution. This typically encompass the central 95% percentile of reference values and the 2.5th and 97.5th percentile as the lower and upper reference limit, respectively.[6] Multiple regression analysis was used to determine the association of BMI, age and FPG values for the total population, and the level of significance was set at P < 0.05.

All standard guidelines for sample collection, processing, storage and handling were strictly adhered to. Instruments were regularly calibrated and recalibrated, and the procedures were carried out with quality control samples in each run.

  Results Top

A total of 332 subjects were recruited into the study and this was made up of 172 (51.8%) males and 160 (48.2%) females, with a mean age of 25.83 years. The lowest and highest FPG values were 2.8 and 6.1 mmol/l, respectively, with a mean value of 4.4 mmol/l. When the reference subjects were divided into males and females, their mean FPG values were 4.4 mmol/l for males and 4.3 mmol/l for females [Table 1]. The FPG reference interval was 3.1–5.8 mmol/l using the non-parametric method. Multiple linear regression analysis showed a positive association of subject's age and BMI with FPG [Table 2].
Table 1: Characteristics and fasting plasma glucose results of study subjects

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Table 2: Test of association between fasting plasma glucose and other factors using multiple linear regressions

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  Discussion Top

Maintaining blood glucose concentration close to the reference interval will prevent or delay diabetic patients from developing complications. This would depend on correctly determined reference interval of FPG for a local population. Reference intervals may vary from population to population, and this may be due to diversity in the genetic constitution, nutritional habits and lifestyles, among others.

The reference interval obtained from this study (3.1–5.8 mmol/l) was different from that stated by the diagnostic kit manufacturer, which was given as 4.2–6.4 mmol/l (Randox glucose kit),[7] and was also different from but close to the results obtained from an earlier study done in Port Harcourt in 2008, which gave a reference interval of 3.0–5.7 mmol/l.[8] That study was also carried out using the glucose oxidase method and the same non-parametric method but with a reference population of 605 apparently healthy individuals. This was about twice the number used in the present study, which may be responsible for the slight and insignificant difference in the reference intervals. Currently, the reference interval in use in our laboratory is 3.9–6.4 mmol/l. The upper reference value of the earlier Port Harcourt study was 5.7 mmol/l while that for this study was 5.8 mmol/l, and this difference was not statistically or clinically significant. This may mean that more than 10 years after the first Port Harcourt study, the population demographics may have changed but not enough to produce a clinically significant change in the reference interval, or that increased awareness of diabetes in Port Harcourt over the last decade may have led to modified lifestyle and nutritional habits of individuals. Increased physical activities due to lifestyle modification will also lead to reduced plasma glucose concentrations,[9] which could be responsible for the clinically insignificant difference of the reference intervals observed in the two Port Harcourt studies.

Factors such as obesity, age or gender have been shown to affect blood glucose concentration in other studies.[10],[11],[12] This was also demonstrated in this study as there was a positive correlation of these factors with FPG, which could explain in part the variations in reference interval among different ethnic groups. To further strengthen this argument is the fact that other studies have shown an age-related increase in glucose values,[11] with significant differences in the mean values of the different age groups when compared.

In this study, when the data were divided into male and female groups, the mean plasma glucose level of the males (4.61) was slightly higher than those of females (4.27), but the difference in the mean values was not statistically significant. The reference interval for the male group was 3.1–5.9 mmol/l while that for the female was 3.1–5.8 mmol/l. These reference intervals and that of the study population were about the same. This is in keeping with other studies that showed that overall, gender had no significant effect on the FPG value.[13]

When the reference intervals from both Port Harcourt studies were compared with that of the kit's manufacturer which was derived from a different reference population, there was significant difference in the values. Diet, environment, genetic factors [15] among others influences blood glucose values which in a way affect the reference intervals of fasting plasma glucose. This may explain the differences in reference values among different ethnic populations, thus further emphasizing the importance of a local population-based reference interval of FPG, even when using the same assay method.[15]

An Iranian study and a study in Kumasi, Ghana, had reference intervals of 3.9–5.6[16] and 3.1–6.3 mmol/l,[17] respectively. Although the Iranian value was close to that determined in this study, the Kumasi value was significantly different from that in this study; however, both studies were done in Sub-Saharan West Africa. Most staple diets in West Africa are rich in carbohydrates such as corn, cassava, yam and coco yam, with different glycaemic indices,[18] and this is prepared in different ways and eaten in different proportions by different ethnic groups. This may influence the plasma glucose concentration, which could also be a reason for the significant difference in the reference intervals observed.

Since reference intervals vary from population to population, and sometimes significantly, there may be need to re-evaluate the universally accepted cut-off values for the diagnosis and management of diabetes mellitus in different populations, especially among populations with significantly low reference values, to further improve on the diagnosis, care and follow-up of diabetics in these populations. This proposal might sound strange, considering the present state of knowledge in diabetology, but needs further consideration and evaluation by the international medical research community.


Based on the Reed's hypothesis, a minimum of 120 subjects can be used for the determination of reference interval of a study population.[6] For this study, a reference population of 332 was used although a larger number would have been preferred.

  Conclusion Top

The reference intervals of the two Port Harcourt based studies were about the same (3.1-5.7 mmol/l and 3.1-5.8 mmol/l) and different form reference interval presently in used in our laboratory (3.9-6.4 mmol/l) and that determined by the manufacturer of the Randox diagnostic kit (4.2-6.4 mmol/l). The use of this new reference interval in our laboratory will be necessary for better management of diabetic patients in our locality.

Financial support and sponsorship


Conflicts of interest

There are no conflicts of interest

  References Top

Mennen LI, Mbanya JC, Cade J, Balkau B, Sharma S, Chungong S, et al. The habitual diet in rural and urban Cameroon. Eur J Clin Nutr 2000;54:150-4.  Back to cited text no. 1
Wild S, Roglic G, Green A, Sicree R, King H. Global prevalence of diabetes: Estimates for the year 2000 and projections for 2030. Diabetes Care 2004;27:1047-53.  Back to cited text no. 2
Nyenwe EA, Odia OJ, Ihekwaba AE, Ojule A, Babatunde S. Type 2 diabetes in adult Nigerians: A study of its prevalence and risk factors in Port Harcourt, Nigeria. Diabetes Res Clin Pract 2003;62:177-85.  Back to cited text no. 3
International Organization for Standardization. Medical Laboratories – Particular Requirements for Quality and Competence. Geneva: International Organization for Standardization; 2012.p36-7.  Back to cited text no. 4
Solberg HE. A guide to IFCC recommendations on reference values. J Int Fed Clin Chem 1993;5:162-5.  Back to cited text no. 5
Horn PS, Feng L, Li Y, Pesce AJ. Effect of outliers and nonhealthy individuals on reference interval estimation. Clin Chem 2001;47:2137-45.  Back to cited text no. 6
Trinder P. Determination of glucose in blood using glucose oxidase with an alternative oxygen acceptor. Ann Clin Biochem 1969;6:24-7.  Back to cited text no. 7
Ejilemele AA, Ojule AC, Orluwene CG. Plasma glucose in a reference population in Port Harcourt. Port Harcourt Med J 2008;2:228-32.  Back to cited text no. 8
Colberg SR, Hernandez MJ. The big blue test: Effects of 14 minutes of physical activity on blood glucose levels. Diabetes Care 2013;36:e21.  Back to cited text no. 9
Broughton DL, Taylor R. Review: Deterioration of glucose tolerance with age: The role of insulin resistance. Age Ageing 1991;20:221-5.  Back to cited text no. 10
Mittendorfer B. Insulin resistance: Sex matters. Curr Opin Clin Nutr Metab Care 2005;8:367-72.  Back to cited text no. 11
Ferrannini E, Natali A, Capaldo B, Lehtovirta M, Jacob S, Yki-Järvinen H. Insulin resistance, hyperinsulinemia, and blood pressure: Role of age and obesity. European Group for the Study of Insulin Resistance (EGIR). Hypertension 1997;30:1144-9.  Back to cited text no. 12
Ghasemi A, Zahediasl S, Azizi F. Reference values for fasting serum glucose levels in healthy Iranian adult subjects. Clin Lab 2011;57:343-9.  Back to cited text no. 13
Geer EB, Shen W. Gender differences in insulin resistance, body composition, and energy balance. Gend Med 2009;6 Suppl 1:60-75.  Back to cited text no. 14
Kuhl J, Hilding A, Ostenson CG, Grill V, Efendic S, Båvenholm P. Characterisation of subjects with early abnormalities of glucose tolerance in the Stockholm Diabetes Prevention Programme: The impact of sex and type 2 diabetes heredity. Diabetologia 2005;48:35-40.  Back to cited text no. 15
Faerch K, Borch-Johnsen K, Vaag A, Jørgensen T, Witte DR. Sex differences in glucose levels: A consequence of physiology or methodological convenience? The Inter99 study. Diabetologia 2010;53:858-65.  Back to cited text no. 16
Arthur FK, Yeboah FA, Nsiah K, Nkrumah PK, Afreh KA, Agyenim-Boateng K. Fasting blood glucose and glycosylated haemoglobin levels in randomly selected Ghanaian diabetic patients-the clinical implications. J Sci Technol 2005;25:13-7.  Back to cited text no. 17
Ogbonna OC, Fadeiye EO, Ikem RT, Oladipo KO, Soyoye DO, Olulana TM, et al. Blood glucose response on consumption of cassava varieties (Garri) in healthy Nigerian subjects. J Nutr Hum Health 2018; 2:22-7.  Back to cited text no. 18


  [Table 1], [Table 2]


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