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ORIGINAL ARTICLE
Year : 2015  |  Volume : 59  |  Issue : 3  |  Page : 204-209  

A reliable and accurate portable device for rapid quantitative estimation of iodine content in different types of edible salt


1 Assistant Professor, Centre for Community Medicine, All India Institute of Medical Sciences, New Delhi, India
2 Senior Programme Officer, Global Alliance for Improved Nutrition, New Delhi, India
3 Associate, Global Alliance for Improved Nutrition, New Delhi, India
4 Professor and Head, Centre for Community Medicine, All India Institute of Medical Sciences, New Delhi, India

Date of Web Publication7-Sep-2015

Correspondence Address:
Chandrakant S Pandav
Room No-31, Centre for Community Medicine, Old OT Block, All India Institute of Medical Sciences, New Delhi - 110 029
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0019-557X.164658

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   Abstract 

Background: Continuous monitoring of salt iodization to ensure the success of the Universal Salt Iodization (USI) program can be significantly strengthened by the use of a simple, safe, and rapid method of salt iodine estimation. This study assessed the validity of a new portable device, iCheck Iodine developed by the BioAnalyt GmbH to estimate the iodine content in salt. Materials and Methods: Validation of the device was conducted in the laboratory of the South Asia regional office of the International Council for Control of Iodine Deficiency Disorders (ICCIDD). The validity of the device was assessed using device specific indicators, comparison of iCheck Iodine device with the iodometric titration, and comparison between iodine estimation using 1 g and 10 g salt by iCheck Iodine using 116 salt samples procured from various small-, medium-, and large-scale salt processors across India. Results: The intra- and interassay imprecision for 10 parts per million (ppm), 30 ppm, and 50 ppm concentrations of iodized salt were 2.8%, 6.1%, and 3.1%, and 2.4%, 2.2%, and 2.1%, respectively. Interoperator imprecision was 6.2%, 6.3%, and 4.6% for the salt with iodine concentrations of 10 ppm, 30 ppm, and 50 ppm respectively. The correlation coefficient between measurements by the two methods was 0.934 and the correlation coefficient between measurements using 1 g of iodized salt and 10 g of iodized salt by the iCheck Iodine device was 0.983. Conclusions: The iCheck Iodine device is reliable and provides a valid method for the quantitative estimation of the iodine content of iodized salt fortified with potassium iodate in the field setting and in different types of salt.

Keywords: ICheck Iodine, India, iodized salt, salt, validation, validity


How to cite this article:
Yadav K, Kumar R, Chakrabarty A, Pandav CS. A reliable and accurate portable device for rapid quantitative estimation of iodine content in different types of edible salt. Indian J Public Health 2015;59:204-9

How to cite this URL:
Yadav K, Kumar R, Chakrabarty A, Pandav CS. A reliable and accurate portable device for rapid quantitative estimation of iodine content in different types of edible salt. Indian J Public Health [serial online] 2015 [cited 2021 Jan 16];59:204-9. Available from: https://www.ijph.in/text.asp?2015/59/3/204/164658


   Introduction Top


Iodine deficiency, one of the most common micronutrient deficiencies, plays a pivotal role in the growth, development, and survival of children. The spectrum of iodine deficiency disorders (IDD) includes and manifests as goiter, cretinism, hypothyroidism, brain damage, abortion, still birth, mental retardation, psychomotor defects, and hearing and speech impairment. [1] IDD constitute the single most important cause of mental impairment worldwide. [2] Salt iodization has been identified as a primary strategy to address IDD globally. [3] Significant progress has been made in the elimination of IDD through salt iodization programs worldwide since the launching of a global movement for universal salt iodization (USI) in 1990s. However, still over 1.88 billion people globally have deficient iodine intake, making them vulnerable to irreversible brain damage. [4] The global iodized salt coverage is 76% and we still have a long way to go before the USI target of 90% is achieved. [5]

The lesson learnt over the past three decades of the salt iodization program has shown that regular monitoring of iodine content of salt from production end to consumer end through the salt distribution chain is critical to ensure the success of the USI program.Iodometric titration is the gold standard test for the estimation of iodine content in salt. Titration requires a laboratory setup and trained laboratory personnel, and cannot be done in a field setting. Although large salt producers and national laboratories usually have the capacity to measure salt iodine levels quantitatively using titration, this may not be the case for small producers or regulatory institutions at decentralized levels, or for surveys collecting household samples. In India, currently a network of salt iodometric titration exists at both the production end (Salt Commissioner Office iodine monitoring laboratory) and the consumer end (Ministry of Health and Family Welfare state-level iodine monitoring laboratories). [6] However, these laboratories are not accessible to the salt producers, traders, and consumers. Continuous monitoring of salt iodization can be significantly strengthened by the use of a simple, safe, and rapid method of salt iodine.

Many salt-testing kits have been developed, but they are constrained by the fact that they can give only qualitative results and the actual quantity of iodine cannot be estimated using these kits. [7],[8] Another method, described in the literature as being accurate and relatively simple, is the WYD iodine checker. [9] Despite the reliability, accuracy, and low cost of this method, it still requires some level of technical skill as solutions need to be prepared and concentrated sulfuric acid needs to be handled. Furthermore, there is a risk of contamination through inappropriate handling of the iodine-containing calibration solution.

A new, portable photometer device, iCheck Iodine has been developed by BioAnalyt GmbH, Teltow, Germany to estimate the iodine content in iodized salt. The objective of the present study was to assess the validity of this new device against the iodometric titration for quantitative measurement of the iodine content of salt. The Regional Office (South Asia), International Council for Control of Iodine Deficiency Disorders (ICCIDD) conducted the validation of iCheck iodine device with the support of the Global Alliance for Improved Nutrition (GAIN).


   Materials and Methods Top


Study setting

Validation of the device was conducted in the laboratory of South Asia regional office of the ICCIDD. The ICCIDD regional laboratory is a reference laboratory for salt and urine iodine estimation in the Southeast Asia Region.

Study duration

The study was conducted from December 1, 2012 to January 31, 2013.

Description of device

The iCheck Iodine device consists of two units, the measuring unit that uses a colorimetric method for iodine content estimation and the disposable reagent vial. The principle of this colorimetric method is based on the reaction of KIO 3 in salt with KI to liberate free iodine, which in turn reacts with starch solution to form a blue-purple complex. The concentration of the blue-colored solution is measured at 565 nm. The device software has an algorithm that converts the absorption units of the blue color into the corresponding iodine concentration in mg/L.

Procedure for estimation of iodine content in salt samples using the device

Reagent vials along with the solution A vial are provided with the device. The reagent vials are activated by injecting 200 μL of solution A in each vial. One gram of salt to be tested is diluted with 4 mL of distilled water. Then, 1 mL of the salt solution is loaded in a syringe and injected in the activated reagent vial. The loaded reagent vial is now rolled between the hands to thoroughly mix the injected salt solution and other contents of the vial. The loaded reagent vial is left to stand for 5 min for completion of the reaction. The iodine concentration is measured with the measuring device and the reading obtained recorded.

Iodometric titration

Iodometric titration was used as the reference method. Salt iodometric titration was carried out as per the standard guidelines. [10]

Methodology

One hundred thirteen (113) salt samples of unknown iodization level and three salt samples with 10 parts per million (ppm), 30 ppm, and 50 ppm iodine were procured from various small-, medium-, and large-scale salt processors from across India were used for validation. These commercially produced salt samples were procured from salt manufacturers through purposive sampling to include refined, washed, and common iodized salt. Also, known-value salt samples of iodine content of 10 ppm, 30 ppm, and 50 ppm and standard iodine solution (prepared using KIO 3 in saline solution) were used for the test runs required for the validation exercise. The validity of the device was assessed using three set of indicators:

  1. Device-specific indicators,
  2. Comparison of the iCheck Iodine device with the iodometric titration,
  3. Comparison between iodine estimation using 1 g and 10 g salt by iCheck iodine.


Device-specific indicators

  1. Linearity: Linearity of the device was assessed by measuring the iodine content of six standard solutions of KIO 3 with iodate concentrations of 0 ppm, 3 ppm, 6 ppm, 9 ppm, 12 ppm, and 15 ppm while maintaining a dilution rate of 1 to 4. All measurements were done in duplicate.
  2. Limit of Detection: Serial dilution of the KIO 3 with known iodine content was tested, and the lowest concentration detected by the device was ascertained.
  3. Intraassay precision: Three salt samples of known iodine concentrations 10 ppm, 30 ppm, and 50 ppm were measured in 10 replicates.
  4. Interassay precision: Three salt samples of known iodine concentrations 10 ppm, 30 ppm, and 50 ppm were measured in five replicates and were repeated over 3 days by one technician.
  5. Interoperator precision: Three salt samples of known iodine concentrations 10 ppm, 30 ppm, and 50 ppm were measured in five replicates on the same day by three different technicians.


Comparison of iCheck Iodine device with the iodometric titration

Two methods were compared using 75 salt samples with unknown iodine quantity using standard methods. Seventy-five salt samples were purposively selected with an assumption of sensitivity and specificity of 85%, expected proportion of salt sample to be adequately iodized to be 71%, [11] and a precision of 15%. This sample size would be sufficient keeping in mind a correlation coefficient of 0.979, [12] power of 90%, and level of significance of 5%.

Comparison between iodine estimation using 1 g and 10 g salt by iCheck Iodine device

Thirty-eight salt samples with unknown iodine quantity were tested by iCheck Iodine device using 1 g and 10 g samples. A sample of 38 was chosen with an assumption of correlation coefficient of 0.5, power of 80%, and level of significance of 5%. A correlation coefficient of 0.5 was assumed as there was no literature available on the correlation coefficient between iodine estimation using 1 g and 10 g of salt.

Statistical analysis

Data were analyzed using SPSS Statistics for Windows, Version 17.0 (SPSS Inc, Chicago). Coefficient of variation was estimated for imprecision in interassay, intraassay, and interoperator imprecision. Correlation between iodine estimation by iCheck Iodine device and iodometric titration and iodine estimation by 1 g and 10 g of salt by iCheck Iodine device was assessed by calculating the correlation coefficient, kappa statistic, and Bland-Altman Plot. [13] As more deviation and disagreement were observed for higher iodine content between the two methods, quadratic spline regression was used to obtain the regression equation. In addition, the paired t-test was applied to ascertain the significance level of difference between the two measurements. The accuracy of the iCheck device was measured in terms of the sensitivity and specificity of the iCheck Iodine device to detect a salt iodization level at a cutoff value of 30 ppm. [14] The significance level was taken as 5%.

Quality assurance process

All the salt samples were tested in duplicate. Levey-Jenning plots were maintained for the internal quality assurance of iodometric titration.


   Results Top


Device-specific indicators

  1. Linearity: The correlation coefficient in the measurement over the iodine content range of 0-75 ppm was 0.998.
  2. Limit of Detection : A solution with a minimum concentration of 1 mg/L iodine could be detected with the iCheck Iodine device.
  3. Intraassay precision : The coefficients of variation of 10 measurements conducted within a day for the iodine concentrations of 10 ppm, 30 ppm, and 50 ppm were 2.8%, 6.1%, and 3.1% respectively.
  4. Interassay precision : The coefficients of variation of 15 measurements conducted over 3 days for the salt with iodine concentration 10 ppm, 30 ppm, and 50 ppm were 2.4%, 2.2%, and 2.1% respectively.
  5. Interoperator precision : The coefficients of variation of 15 measurements conducted by three technicians on the same day for the iodine concentration 10 ppm, 30 ppm, and 50 ppm were 6.2%, 6.3%, and 4.6% respectively.


Comparison of iCheck Iodine device with the iodometric titration

The coefficients of variation for duplicate analysis for iCheck Iodine device were 1.5% and 1.4% for iodometric titration. [Figure 1] depicts the scatter plot of iodine content of salt samples by iodine iCheck device against iodometric titration. The correlation coefficient between measurements by two methods was 0.934 and the regression equation with iodine content by iodometric titration as independent variable (X) and iCheck Iodine readings as dependent variable (Y) was: Y = 0.127 + 1.209 * x + 0.005 * x*x. The upper limit of agreement between two methods was 6.7 ppm and the lower limit of agreement was -7.8 ppm and the mean difference between two methods was -0.54 ppm [Figure 2]. The paired t-test between iodine content of salt samples by iCheck and iodometric titration was not significant (P valve = 0.20). [Table 1] depicts concordance between iodine content estimation in salt by iCheck Iodine method and iodometric method. The sensitivity of the iCheck Iodine device against gold-standard iodometric titration was 87.8%, and the specificity was 94.1%. The kappa statistic between two methods to detect a salt iodization level above or below 30 ppm was 0.813.
Figure 1: Scatter plot of iodine content of iodized salt sample by iCheck Iodine device against iodometric titration

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Figure 2: Bland-Altman plot for comparison of salt iodometric titration and iodine estimation by the iCheck Iodine device

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Table 1: Concordance between iodine content estimation in salt by iCheck Iodine method and iodometric titration

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Comparison between iodine estimation using iCheck Iodine 1 g and 10 g salt

The correlation coefficient between measurements by using 1 g and 10 g of salt was 0.983. The regression equation with iodine content of 1 g of salt as dependent variable (Y) and 10 g of salt as independent variable (X) was: Y = 0.9649 x + 0.1988. The upper limit of agreement between the two methods was 5.0 ppm and the lower limit of agreement was -5.3 ppm and the mean difference between two methods was -0.1 ppm. The paired t-test between the iodine content of salt samples by iCheck device using 1 g and 10 g of salt was not significant (P valve = 0.73).


   Discussion Top


The present study successfully demonstrates that using the iCheck Iodine device is a reliable and valid method for quantitative estimation of the iodine content of iodized salt fortified with potassium iodate in different types of salt including refined, washed, and common (unrefined) salt in the field setting. The device can be of potential use for monitoring salt iodization levels from the production end to the supply chain. However, its utility at the consumption end needs to be determined. The correlation and agreement between the reference iodometric titration method and the iCheck Iodine device were good, with the portable device measuring on an average only 0.5 ppm lower than the titration method. The observed precision levels were satisfactory and within acceptable limits, the intraassay imprecision was below 6%, the interassay imprecision was below 2.4%, and the interoperator imprecision was 6.3%. An earlier study, in which the iCheck Iodine device was validated, found the correlation coefficient between the iCheck Iodine device and iodometric titration to be 0.978, which is slightly better than the current study that reports a correlation coefficient of 0.934. That study also reported better intraassay and interassay precision values of 0.5-0.9% and 1.5-2.5% respectively. However, the better results could be due to better quality of salt samples, as the quality of salt samples has been shown to affect the performance of the device.

Studies have validated other methods of iodine estimation in salt containing iodate. Validation of rapid testing kits found them to be unreliable in quantitative estimation of iodine content of salt. Studies have also found them to be of low specificity. [15] A recent study has reported the sensitivity of the rapid test kit to be 99%; however, the specificity was only 66%. [16] Other rapid diagnostic devices have been also reported. One such device was the WYD iodine checker. While the correlation coefficient of WYD iodine checker with iodometric titration was found to be 0.850, the correlation coefficient of iCheck Iodine was 0.934. Similarly, the interassay precision of the WYD device was found to be 3.4-4.7% as against 2.1-2.4% of the iCheck Iodine device. The intraassay precision of the iCheck iodine device was also better (2.8-6.1%) as compared to the WYD checker (2.1-10.3%). The WYD checker, though reliable, accurate, and of low cost, also requires the preparation of reagents involving concentrated sulfuric acid, limiting its use in the field. A similar limitation was found in a spectrophotometer validated for the estimation of iodine content in salt in India. [17] The iCheck Iodine device differs significantly from the other devices in terms of the provision of preloaded reagent vials (the reagent has to be constituted for WYD iodine checker, leading to the handling of hazardous chemicals such as sulfuric acid) and also the significant reduction in time required to test one sample.

The main strength of this study was that the different type of salt produced commercially from different salt-producing centers was used for the study. The limitations of the study included that only salt fortified with potassium iodate was used, thus salt fortified with potassium iodide may or may not give similar accurate and valid results. Also, for interobserver variation, only two trained laboratory technicians were studied. The interobserver variation might be higher than observed in the study, especially when the device is used by less experienced and untrained personnel in the field setting. Instructions for the safe and proper disposal of syringes used during the iodine estimation by iCheck Iodine device are required. The manufacturer should include instructions for disposal of used syringes using needle cutter and syringe destroyer.

Future research is warranted to systematically document the effects of the physicochemical characteristics of salt and interfering substances [Fe(III)+ or other strong, oxidizing agents]. Also, studies on salt fortified with potassium iodide and on iodine estimation in double-fortified salt are required. It may be worthwhile to conduct a detailed cost analysis of the device and explore possibilities of rationalizing the cost of the iCheck Iodine device and its reagent vials to enable its large-scale adoption.


   Conclusion Top


The iCheck iodine device is reliable and provides a valid method for the quantitative estimation of the iodine content of iodized salt fortified with potassium iodate and in different types of salt (refined, washed, and common). This quick and portable device can help in strengthening the monitoring of the level of salt iodization at the production end and the supply chain, especially in settings where access to iodometric titration facilities is limited.

Acknowledgment

We acknowledge the support of Global Alliance for Improved Nutrition (GAIN) in conducting this study. We also acknowledge the laboratory personnel from the regional office of the International Council for Control of Iodine Deficiency Disorders (ICCIDD).

Financial support and sponsorship

This research was funded by a grant from Global Alliance for Improved Nutrition (GAIN), Geneva.

Conflicts of interest

There are no conflicts of interest.

 
   References Top

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Dearth-Wesley T, Makhmudov A, Pfeiffer CM, Caldwell K. Fast and reliable salt iodine measurement: Evaluation of the WYD Iodine Checker in comparison with iodometric titration. Food Nutr Bull 2004;25:130-6.  Back to cited text no. 9
    
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UNICEF, Ministry of Health and Family Welfare, Government of India. Coverage Evaluation Survey 2009 All India Report. New Delhi: UNUCEF; 2010. Available from: http://www.unicef.org/india/1_-_CES_2009_All_India_Report.pdf. [Last accessed on 2014 Jul 5].  Back to cited text no. 11
    
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    Figures

  [Figure 1], [Figure 2]
 
 
    Tables

  [Table 1]


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