Validation of spot-testing kits to determine iodine content in salt

Iodine deficiency disorders are a major public health problem, and salt iod ization is the most widely practised intervention for their elimination. Fo r the intervention to be successful and sustainable, it is vital to monitor the iodine content of salt regularly. Iodometric titration, the traditiona l method for measuring iodine content, has problems related to accessibilit y and cost. The newer spot-testing kits are inexpensive, require minimal tr aining, and provide immediate results. Using data from surveys to assess th e availability of iodized salt in two states in India, Madhya Pradesh and t he National Capital Territory of Delhi, we tested the suitability of such a kit in field situations. Salt samples from Delhi were collected from 30 sc hools, chosen using the Expanded Programme on Immunization (EPI) cluster sa mpling technique. A single observer made the measurement for iodine content using the kit. Salt samples from Madhya Pradesh were from 30 rural and 30 urban clusters, identified by using census data and the EPI cluster samplin g technique. In each cluster, salt samples were collected from 10 randomly selected households and all retailers. The 15 investigators performing the survey estimated the iodine content of salt samples in the field using the kit. Ail the samples were brought to the central laboratory in Delhi, where iodine content was estimated using iodometric titration as a reference met hod. The agreement between the kit and titration values decreased as the nu mber of observers increased. Although sensitivity was not much affected by the increase in the number of observers (93.3% for a single observer and 93 .9% for multiple observers), specificity decreased sharply (90.4% for a sin gle observer and 40.4% for multiple observers). Due to the low specificity and resulting high numbers of false-positives for the kit when used by mult iple observers ("real-life situations"), kits were likely to consistently o verestimate the availability of iodized salt. This overestimation could res ult in complacency. Therefore, we conclude that until a valid alternative i s available, the titration method should be used for monitoring the iodine content of salt at all levels, from producer to consumer,to ensure effectiv eness of the programme.