Plasma glucocorticoid (CORT) levels are one measure of stress in wildlife and give us insight into natural processes relevant to conservation issues. Many studies use total CORT concentrations to draw conclusions about animals' stress state and response to their environment. However, the blood of tetrapods contains corticosteroid-binding globulin (CBG), which strongly binds most circulating CORT. Only free CORT (CORT not bound by CBG) leaves the circulation and exerts biological effects on CORT-sensitive tissues. Measuring free CORT concentrations provides insight to an animal's stress response that cannot be revealed by simply measuring total CORT. To calculate free CORT concentrations in plasma or serum samples, one needs three measurements: the binding affinity of CBG for CORT (which varies by species), the total CORT concentration in the sample and the maximum corticosteroid binding capacity (MCBC) of CBG in the sample. Here, we detail the measurement of CBG binding capacity. We compare and contrast the three main methods to measure MCBC: charcoal, cell harvester and dialysis. Each is defined by the means by which free and bound CORT are separated. We weigh the relative merits and challenges of each. We conclude that sample volume, species and taxon binding specificity, and availability of equipment are the primary considerations in selecting the appropriate separation method. For most mammals, the charcoal method is recommended. For birds, the harvester method has critical advantages over the charcoal method. The dialysis method is widely regarded as the gold standard and has lower equipment costs but is more time-intensive and costly in terms of radioactive isotope needed and is less suited to processing large numbers of samples. The binding capacity of CBG varies tremendously within and among the bird and marine mammal species studied, and we discuss the implication of this variation for understanding the role of stress in wildlife.

中文翻译：

---

游离糖皮质激素的测量：量化皮质类固醇的结合能力及其在哺乳动物和鸟类内部以及之间的变异。

血浆糖皮质激素（CORT）水平是衡量野生生物压力的一种手段，使我们能够深入了解与保护问题相关的自然过程。许多研究使用总CORT浓度得出有关动物应激状态和对环境的反应的结论。但是，四足动物的血液中含有与皮质类固醇结合的球蛋白（CBG），可与大多数循环CORT牢固结合。只有游离的CORT（不受CBG束缚的CORT）离开循环并在对CORT敏感的组织上发挥生物学作用。测量游离CORT浓度可以洞察动物的应激反应，而仅通过测量总CORT便无法揭示这种应激反应。要计算血浆或血清样品中的游离CORT浓度，需要进行三项测量：CBG对CORT的结合亲和力（随物种而异），样品中的总CORT浓度和样品中CBG的最大糖皮质激素结合能力（MCBC）。在这里，我们详细介绍了CBG结合能力的测量。我们比较并对比了三种测量MCBC的主要方法：木炭，细胞收集器和透析。每种方法均通过将游离和结合的CORT分开的方式定义。我们权衡每个相对优缺点。我们得出结论，样品量，种类和分类群结合特异性以及设备的可用性是选择合适的分离方法的主要考虑因素。对于大多数哺乳动物，建议使用木炭方法。对于鸟类，收割机方法比木炭方法具有关键优势。透析方法被广泛认为是金标准，具有较低的设备成本，但在所需的放射性同位素方面更耗时且成本较高，并且不适合处理大量样品。在研究的鸟类和海洋哺乳动物物种内部和之中，CBG的结合能力差异很大，我们讨论了这种变异对理解压力在野生生物中的作用的影响。