Indian Pediatrics 2000;37: 182-186
Validation of Spot Testing Kit in the Assessment of Iodine Content of Salt: A Multi-Centric Study
Umesh Kapil, S.N. Dwivedi, S. Seshadri, S.S. Swami, Beena, B.P. Mathur, T.D. Sharma, K. Khanna, R.S. Raghuvanshi, M. Tandon, P. Pathak, R. Pradhan
From the Departments of Human Nutrition and Biostatistics, All India Institute of Medical Sciences, New Delhi 110 029, India; Department of Foods and Nutrition, M.S. India University, Baroda, Gujarat, India, Department of Preventive and Social Medicine, S.P. Medical College, Bikaner, Rajasthan, India. Mamta Samajik Sanstha, 53-C, Rajpur Road, Dehradun, India. Department of Preventive and Social Medicine, Medical College, Jhansi, U.P., India; Health and Family Welfare Training Center, Kangra, H.P., India; Institute of Home Economics, New Delhi, India; and Department of Foods and Nutrition, GBPUAT, Pantnagar, District Udham Singh Nagar, U.P. India.
Reprint requests: Dr. Umesh Kapil, Additional Professor, Department of Human Nutrition, All India Institute of Medical Sciences, New Delhi 110 029, India.
Manuscript received: June 3, 1999;
Initial review completed: July 26, 1999;
Revision accepted: August 23, 1999
The use of iodized salt has been evolved as a major strategy to combat Iodine Deficiency Disorders (IDDs). According to Prevention of Food Adulteration Act (PFA), in India the salt should contain at least 15 ppm iodine to provide the normal requirements of 150 micrograms per day to the population. It is usually necessary to iodize the salt at higher levels (30 ppm or more) to compensate for the loss of iodine during storage and distribution. For the successful implementation of salt iodization program, monitoring the quality of salt is essential. The iodine content of iodized salt is estimated routinely by the standard "Iodometric Titration" (IT) method in the laboratory(1). However, a simple kit has been developed in India for `on-the-spot' estimation of iodine content. In the spot testing method, estimation of iodine content in the salt is done with the help of a standard starch solution provided in the Spot Testing Kit (STK) . The STK has been advocated as a method for semi-quantitative estimation of iodine in salt(2,3). The present multicentric study was conducted with the objective to assess the validity of the STK (against the IT) for the semi-quantitative estimation of iodine content of salt.
Material and Methods
The present research study was conducted at randomly selected eight centers in the country, namely, Baroda (Gujarat), Bikaner (Rajasthan), Dehradun and Jhansi (Uttar Pradesh), Kangra (Himachal Pradesh), New Delhi (IHE), Udham Singh Nagar (Uttar Pradesh) and New Delhi (AIIMS). The research project was co-ordinated by the Department of Human Nutrition, All India Institute of Medical Sciences, New Delhi.
Calculation of Sample Size
With an anticipated agreement between IT method and STK method as 85%(4) and assuming ideal agreement as 80%, level of confidence of 95% and power of the study 95%, the minimum sample size of 660 was calculated. To have higher precision, it was decided to undertake a sample size of 700 at each center.
Collection of Salt Samples
Each participating center randomly selected one rural block at least 30 km away from the district headquarters. In the block, one high school was further randomly selected keeping in view the operational feasibility. In each school, 800 children were randomly included for the study (expecting that 100 children may not bring salt sample or may bring less quantity of salt). These school chidren were briefed about the study objectives and were given auto-seal polythene pouches (airtight moisture proof) with an identification proof) with an identification slip. They were requested to bring about 20 g of salt (four teaspoonfuls) from their family kitchen. Uniform protocol was utilized at all the eight centers.
Estimation of Iodine Content of Salt
Each salt sample was given an identification number and was analyzed for its iodine content by standard IT method by a research scientist. Subsequently, the salt samples were recorded and tested again for the iodine content by the STK method (as the research scientist was given recoded samples, she/he was not aware about the results of IT method). The results obtained by the two methods were compared. The results of IT method were used as reference standard for comparison.
The procedure for use of STK was as follows: The STK consists of a test solution (A) and a recheck solution (B). One drop of test solution (A) was added to about 5 g of salt sample. The change in color of the salt sample ranged from white (no change in color) to dark violet depending on the iodine content of salt, i.e., nil, less than 15 ppm and 15 ppm and above, which was compared with the standard color chart provided with the STK. If on addition of test solution (A), no change in color of salt was observed, the recheck solution (B) was added. This was done to make the salt medium acidic, in case the salt had alkaline constituents, subsequently, the test solution (A) was added again. The intensity of the blue color was directly proportional to the iodine content of salt.
For the purpose of present study, the results of iodine content of salt as analyzed by STK method were evaluated under two categories: (i) salt samples with less than 15 ppm of iodine, and (ii) salt samples with 15 ppm and more of iodine.
The Sensitivity, Specificity, Positive Predictive value (PPV) and Negative Predictive Value (NPV) of the STK method in relation to the IT was calculated, using the standard formulae. The sensitivity of STK was calculated as the percentage of salt sample truly identified by STK as having 15 ppm and more of iodine, i.e., the ability of STK to identify correctly all those salt samples which had more than 15 ppm of iodine content (i.e., true positive). Ninety per cent sensitivity of STK means that 90 per cent of the salt samples (having more than 15 ppm iodine) screened by the STK test will give a "true positive" result and the remaining 10 per cent as "false negative" result, i.e., wrongly classified as having less than 15 ppm iodine.
Similarly, the specificity was calculated as the percentage of salt samples truly identified by the STK as having less than 15 ppm of iodine content (usually known as negatives) of those negatives identified by IT method. Specificity was calculated as the ability of a test (STK) to identify those salt samples which do not have salt iodine content more than 15 ppm, that is,"true negative". A 90 per cent specificity of STK means that 90 per cent of the salt samples having iodine content less than 15 ppm will give "true negative" results by the STK test and remaining 10 per cent of the salt samples which have iodine content less than 15 ppm, will be wrongly classified by the STK test as having more than 15 ppm iodine content. The PPV was determined as the percentage of salt samples with iodine content of 15 ppm or more indicated by IT out of those identified by the STK. On the other hand, the NPV was determined as the percentage of salt samples with less than 15 ppm iodine content indicated by the IT method out of those identified by the STK.
The internal quality control programme for iodometric titration was conducted at each center with each batch of salt samples analyzed.
A total of 6302 salt samples were collected and analyzed at eight different centers in the country out of which 18.8% samples were cystalline (Table I). Table II reveals the results of salt iodine content by IT and STK.
The results of sensitivity and specificity of STK against IT method assessed at eight different research centers in the country is shown in Table III. The highest sensitivity of STK was reported by district Udham Singh Nagar (95.5%).
The PPV of the various research centers ranged from 58.4% to 100.0% and the NPV ranged from 26.8% to 90.4% (Table III).
Table I__Total Number of Salt Samples Analyzed at Different Centers
Table II__Iodine Content of Salt Samples by Both IT ans STK Methods at Different Centers
a* = Iodine content 15 ppm and more by IT method as well as STK method;
b* = Iodine content less than 15 ppm by IT method but 15 ppm and more by STK method;
c* = Iodine content 15 ppm and more by IT method but less than 15 ppm by STK method;
d* = Iodine content less than 15 ppm by IT method as well as STK method.
Table III__ Sensitivity, Specificity, PPV and NPV of STK against Iodometric Titration Method at Different Centers
* PPV_Positivie Predictive Value;
** NPV_Negative Predictive Value.
Iodometric titration is the recommended method for assessment of iodine content of salt. However, it requires a laboratory set up and trained manpower. The STK method is simple and can be used by peripheral health functionaries, school teachers, community leaders and traders with elementary/basic training. The STK method is recommended by the Government of India for qualitative assess-ment of iodine in salt to be used routinely by the government and non-government functionaries. The present study revealed that STK method had an average sensitivity of 89.8% and specificity of 65.6%.
A study conducted by Indian Institute of Health Management Research, Jaipur in 1997 on salt samples collected from seven different states also reported the sensitivity of STK between 77% to 99.2% and specificity from 23.7 to 77.2% in different states(2).
The present study revealed that for semi-quantitative estimation of iodine in salt, STK method has a high sensitivity but low specificity. These findings suggest that STK method can be used for semi-quantitative estimation of iodine content of salt only for the purpose of monitoring the quality of salt available to the community.
We would like to thank Dr. Sheila Vir, Project Officer, Nutrition, India Country Office, UNICEF for her kind support and guidance during the different stages of the research study. We are grateful to Mr. R. Prakash, Salt Commissioner for the help and encouragement extended during the study. We would also like to thank school principals, teachers and students for their kind co-operation in collection of salt samples for the study.
1. Tyabji R, Karmarkar MG, Pandav CS, Carriere CR, Acharya S. Estimation of iodine content in iodated salt. In: A Hand Book of Monitoring and Quality Control. New Delhi, UNICEF ROSCA Publication 1990; pp 18-32.
2. Gupta SD, Gupta H, Kanjilal B, Kumar D, Sharma S, Rao PH, et al. Evaluation of Universal Salt Iodization in India_A Mid Term Evaluation Study. Indian Institute of Health Management Research, Jaipur, India, 1998 (personal communication).
3. Kapil U, Bhanti T, Saxena N, Nayar D, Dwivedi SN. Comparison of Spot Testing Kit in the estimation of iodine content in salt. Indian J Physiol Pharmacol 1996; 40: 276-280.
4. Bhasin SK, Dubey KK. Validity of spot testing kit for estimation of iodine content of salt under field conditions in national capital territory of Delhi. Indian J Matern Child Health 1997; 8: 42-43.