The activated protein C resistance (APCR) test is a coagulation test used in the evaluation and diagnosis of activated protein C (APC) resistance, a form of hypercoagulability.^[1][2] Hereditary APC resistance is usually caused by the factor V Leiden mutation, whereas acquired APC resistance has been linked to antiphospholipid antibodies, pregnancy, and estrogen therapy.^[3][4][5][6] APC resistance can be measured using either an activated partial thromboplastin time (aPTT)-based test or an endogenous thrombin potential (ETP)-based test.^[5][4][2]

The aPTT-based APC resistance test involves a modified aPTT test performed in the presence and absence of activated protein C (APC).^[1][5] The ratio of these aPTT values is calculated and is called the APC sensitivity ratio (APCsr) or simply APC ratio (APCr).^[1][5] This ratio is inversely related to the degree of APC resistance.^[7] The ETP-based APC resistance test involves the addition of APC to a thrombin generation assay (TGA).^[5] This results in an inhibition of thrombin generation as measured by reduction of the endogenous thrombin potential (ETP; area under the thrombin generation curve).^[5] The result is expressed as a normalized APC sensitivity ratio (nAPCsr), which corresponds to the ratio of the ETP measured in the presence and absence of APC divided by the same ratio in reference plasma.^[5] nAPCsr values range from 0 to 10.^[5] Opposite to the case of the APCsr with the aPTT-based APC resistance test, higher nAPCsr values indicate greater APC resistance.^[5][8] This is the result of the fact that APC prolongs the aPTT but inhibits thrombin generation.^[8]

Whereas the aPTT-based APC resistance test only measures the initiation phase of coagulation, the ETP-based test is a global assay and measures the initiation, propagation, and termination phases of coagulation.^[5][9] The initiation phase accounts for less than 5% of total thrombin generation, making aPTT-based tests poorly indicative of hypercoagulability in general.^[10][11] The aPTT-based assay is more sensitive to levels of prothrombin and factor VIII, whereas the ETP-based test is more sensitive to levels of tissue factor pathway inhibitor (TFPI) and protein S.^[5] The ETP-based test has traditionally been performed using methods such as the calibrated automated thrombogram (CAT) and has been limitedly available due to its technical difficulty.^[2] Recently however, a fully automated commercial test system called the ST Genesia has been introduced, and it has been said that this should allow for adoption of TGAs and ETP-based APC resistance tests in routine clinical settings.^[2][12]

Estrogens are well known to increase APC resistance, which has been described as acquired APC resistance.^{[2][5][4][13][14]} However, the aPTT-based APC resistance test is much less sensitive to the procoagulatory effects of estrogens than is the ETP-based test.^{[13][14][5][4][2][15]} Pregnancy^[7] and ethinylestradiol (EE)-containing combined birth control pills increase APC resistance as measured by either the aPTT- or ETP-based test.^[4][5][15] EE-containing birth control pills show different degrees of influence on the ETP-based test depending on the progestin, which may be due to varying degrees of androgenic antagonism of ethinylestradiol-mediated procoagulation.^[5][4] In contrast to EE-containing birth control pills, studies have not found increased APC resistance with menopausal hormone therapy or with estetrol- or estradiol-containing birth control pills using the aPTT-based test, though increased APC resistance has been shown with the ETP-based test.^[14] The increase in APC resistance is much greater with oral estrogens than with transdermal estradiol.^[14] Increased APC resistance with both the aPTT-based and ETP-based tests has been observed with feminizing hormone therapy in transgender women, which involves higher doses of estradiol than are used in other contexts.^[16][17] EE produces a much stronger increase in APC resistance than does estradiol.^[18][17] In relation to this, ethinylestradiol is associated with a higher risk of venous thromboembolism (VTE) than is estradiol.^[18][19][20]

The aPTT-based APC resistance test was developed in 1993, while the ETP-based test was developed in 1997.^[5] For many years, the ETP-based APC resistance test suffered from a lack of standardization which hampered study-to-study comparison.^[21] By 2020 however, a validated methodology was developed aiming to propose a standardized and harmonized scale for ETP-based APC resistance, the normalized activated protein C sensitivity ratio (nAPCsr).^[21]