Screening for hypothyroidism: Age of sample and chance of its falling below the cut-off point

Abstract

The 3 largest hypothyroid screening programmes in Australia use a thyroxine (T 4 ) assay on dried blood spots as the primary screen, backed up by a thyrotropin (TSH) measurement on samples whose T 4 value falls into the lowest 10% of any batch. It was recently shown that recoverable T 4 declines upon storage of blood spots at high temperatures or in direct sunlight. In N.S.W. we have recorded the age, from collection to receipt in the laboratory, of samples, and the age of those samples falling into the lowest 10% of each batch. Twenty-five percent of samples arrived within 1-2 d of collection, but only 1.6%, of these fell into the lowest 10% of the batch in which they were sampled. By contrast almost 10% of samples arrived more than 10 d after collection, and 27%, of these fell into the lowest 10% of the batch. The range of results for one week is shown in the table. Age of sample (days) 1-2 3 4 5-6 7-8 9 0 10 Per cent of total (n = 1501) 25 26 19 17 7 10 Per cent below cut-off 1.6 8 12 16 18 27 Of 27 detected cases of hypothyroidism, only 2 fell into the group whose sample arrived in 1 4 d. Wc concluded that delay in remitting samples to the laboratory seriously distorts results and may lead to missed cases of hypothyroidism particularly in those samples submitted promptly, in programmes with a primary T 4 screen.