Suboptimal Sensitivity

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TSH and Tg assay development share many analogies concerning their quests for improved sensitivity. The clinical utility of TSH measurement has been dramatically enhanced over the last decade by the optimization of IMA methodology, leading to a 100-fold improvement in TSH assay functional sensitivity. Unfortunately, the development of more sensitive Tg assays is still in its infancy. Currently, Tg assay functional sensitivity ranges between 0.3 and 2.5 mg/L (Figure 18.1). As shown in Figure 18.2, a 100-fold improvement in Tg assay functional sensitivity (~0.01 mg/L) would dramatically improve the diagnostic sensitivity of measuring Tg during THST. Specifically,Figure 18.2 shows that there is a striking linear relationship between the serum Tg nadir measured without TSH stimulation in the first postoperative year and long-term (median 8-year) recurrence risk [16]. This suggests that less than 1% of patients with serum Tg below 0.1 mg/L during their first postoperative year would suffer a recurrence [16,17].

As Tg assay sensitivity becomes a marketing issue, there will be increasing commercial pressure for the diagnostic kit manufacturers to make unrealistic claims for the sensitivity of their tests. The guidelines state that the functional sensitivity of a Tg assay should be determined from the lowest Tg value that can be measured with 20% between-run coefficient of variation, using TgAb-negative human sera measured across a 6- to 12-month period and using different lots of reagent [10]. It is critical that Tg method comparisons are made on the basis of functional sensitivity and that descriptive terms such as "ultrasensitive" and "supersensitive" are not used for marketing purposes [10].

Unfortunately, even with Tg assay functional sensitivity determined according to the standardized protocol, the absolute (mg/L) functional sensitivity of different methods cannot be compared because of method biases (Figure 18.1A). The ability to detect small amounts of tumor is related to the degree of discrimination between the assay lower reference limit for normal euthyroid subjects and its functional sensitivity. Currently, there is very little discrimination between the lower reference limit for normal euthyroid subjects and the functional sensitivity limits of current methods, as

Figure 18.2 The relationship between cumulative percent recurrence in a cohort of 278 papillary thyroid cancer patients followed over a median of 8 years and the median group serum Tg nadir value reported (in the absence of TSH stimulation) during the first year following thyroidectomy. G1 had serum Tg nadirs between 1.0 and 1.9 mg/L; G2 had serum Tg nadirs between 2.0 and 4.9 mg/L;G3 had serum Tg nadirs between 5 and 10 mg/L; G4 had serum Tg nadirs above 10 mg/L [16].

Figure 18.2 The relationship between cumulative percent recurrence in a cohort of 278 papillary thyroid cancer patients followed over a median of 8 years and the median group serum Tg nadir value reported (in the absence of TSH stimulation) during the first year following thyroidectomy. G1 had serum Tg nadirs between 1.0 and 1.9 mg/L; G2 had serum Tg nadirs between 2.0 and 4.9 mg/L;G3 had serum Tg nadirs between 5 and 10 mg/L; G4 had serum Tg nadirs above 10 mg/L [16].

shown in Figure 18.1A. In fact, serum Tg was paradoxically undetectable for some subjects by some methods. A new Tg assay nomenclature system has recently been proposed whereby each "generation" of assay would display an order of magnitude greater discrimination between the lower reference limit and functional sensitivity [13]. Under this proposal, current Tg assays would be defined as "first generation". Ideally, third generation Tg assays, characterized by functional sensitivities in the 0.001 to 0.01 mg/L range would dramatically improve the clinical sensitivity of Tg testing during THST (Figure 18.2) [18,19].

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