Comparison of some indicators of the thyroid gland functional activity in simultaneous testing of capillary and venous blood

Abstract

Background. In clinical practice it is often impossible to draw blood from peripheral veins for laboratory testing. At the same time the problem of comparison of venous and capillary blood parameters is still insufficiently studied. Aims. To compare 5 laboratory indicators of the thyroid gland function FT3 (free T3), FT4 (free T4), TgAb (thyroglobulin antibodies), TPOAb (thyroid peroxidase antibodies), and TSH in order to assess deviations of these parameters in venous against capillary blood. Materials and methods. The study enrolled 22 clinically healthy patients of both sexes who had their venous and capillary blood samples simultaneously collected in compliance with pre-analytical rules. The tests were performed within three hours of sample collection on the analyzer Roche Cobas e601. Mean values were calculated for all parameters using a 95% confidence interval based on bootstrap. Differences between paired values of venous and capillary blood parameters were calculated and expressed in relative units (%). Wilcoxon test and correlation analysis were used to compare dependent samples. Results. A statistically significant increase in capillary against venous samples was shown for FT3 (mean deviation 3.11; p 0.001), FT4 (3.89%, p 0.001), TgAb (2.73%; p = 0.041) and TPOAb (6.75%; p 0.001), and a decrease for TSH (-4.3%; p 0.001). A 5%-deviation is in most cases clinically insignificant for diagnosis and choosing a treatment. All the capillary blood parameters under study strongly correlate (r 0.75) with the same parameters of the venous blood: r = 0.971 for FT3; r = 0.993 for FT4; r = 0.958 for TgAb; r = 0.836 for TPOAb; r = 0.995 for TSH. Conclusions. Thus, FT3, FT4 and TSH can be determined in the capillary blood without affecting accuracy and precision of clinical evaluation, while for TgAb and TPOAb these deviations may be significant only when the results are in upper borderline values.

Keywords

• thyroid function tests
• clinical laboratory services
• comparative study
• venous blood