This study evaluated the relationship between APTT and anti-Xa activity, and the cause of the discrepancy was investigated in samples from critically ill patients. The percentage of samples in the concordant groups was 45%, indicating that more than 50% of the samples did not correspond to the measured APTT level to the theoretical APTT levels calculated by anti-Xa activity. The lower group showed higher coagulation factor activity levels, especially factor VIII, and were considered to have pseudo-heparin resistance. In the pseudo-heparin resistance group, apparent heparin overdoses were frequently observed.

In the acute phase of critical illness, coagulopathy induced by bleeding, hepatic failure, and disseminated intravascular coagulation are frequently observed [21,22,23,24]. In these disorders, coagulation and fibrinolytic reactions occur, and coagulation factor activity levels decrease due to their consumption during coagulation activation. APTT was prolonged in these patient samples. Moreover, this study observed coagulation and fibrinolysis activation because an increase in FDP and d-dimer levels and APTT prolongation were confirmed (Table 2). Importantly, a prolonged APTT relative to anti-Xa activity is frequently observed. In addition, severe antithrombin deficiency induces heparin resistance, which is detected as a low anticoagulation effect relative to the amount of heparin administration [25,26,27]. Notably, decreased antithrombin activity levels are frequently observed in the acute phase in critically ill patients [23, 24]. Although moderate decreases in antithrombin activity levels were frequently observed in the upper group, there was no severe deficiency in antithrombin samples in this study (Table 2). Furthermore, large amounts of heparin are not administered to patients with severe deficiencies due to severe coagulopathy. In the upper group, the discrepancy is due to lower coagulation factor activity levels arising from the consumption of these factors; this is classified as a type of coagulopathy.

Fibrinogen and coagulation factor VIII are acute-phase proteins that increase gradually in response to inflammation [28]. Fibrinogen synthesized in the liver is mainly upregulated by interleukin 6 released after inflammation [28]. The synthesis of coagulation factor VIII is observed not only in hepatocytes, but also in the kidneys, endothelial cells, and lymphatic tissue [29, 30] and is upregulated by inflammatory stimulation [31]. In the present study, both fibrinogen and factor VIII levels were markedly elevated, and these elevations, derived from the inflammatory stimulation of critically ill patients, would affect the discrepancy between APTT and anti-Xa activity in the lower group. In the lower group, when anti-Xa activity reached sufficient levels for anticoagulation, APTT was not prolonged. Thus, this group was classified as pseudo-heparin resistant. If UFH is monitored only using APTT in patients with pseudo-heparin resistance, the over-administration of UFH will be induced.

In the lower group (pseudo-heparin resistance type), the discrepancy between APTT and anti-Xa activity might be affected by UFH clearance in vivo. UFH is heterogeneous in terms of molecular weight, pharmacokinetic properties, and anticoagulation activities [32]. The range of UFH molecular weight spans from 3000 to 30,000 Da, and in vivo clearance of UFH is affected by molecular weight [32]. The clearance of high-molecular-weight heparin is greater than that of low-molecular-weight heparin. Furthermore, the clearance of low-molecular-weight heparin is more sensitive to renal function than that of high-molecular-weight heparin [33]. Moreover, the anticoagulation activity of low-molecular-weight heparin is not reflected in APTT [32]. Therefore, in the lower group (pseudo-heparin resistance type), the increased concentration of low-molecular-weight heparin due to differences in in vivo clearance by molecular weight may have contributed to the discrepancy between APTT and anti-Xa activity.

APTT is used to screen for the intrinsic and common pathways involved in the coagulation cascade. However, the effect on APTT prolongation differs for each coagulation factor [34,35,36]. Lawrie et al. investigated the sensitivity of the APTT reagent to each factor based on the Clinical and Laboratory Standard Institute guidelines and reported that coagulation factors that show a clotting time in the upper limit of the reference interval were factors VIII, IX, XI, and XII at levels of 31%, 31%, 52%, and 14%, respectively [37]. Another study indicated that APTT prolongation was observed for factor V activity levels < 45%, factors II and XI activity levels < 40%, and factors I, V, VII, VIII, IX, and XII activity levels < 25% [36]. This indicated that a difference in the sensitivity for each coagulation factor, including extrinsic and common pathways similar to the intrinsic pathway, existed in APTT reagents. Therefore, APTT prolongation would differ even if the activity levels were the same among the coagulation factors [36, 37]. This study confirmed that coagulation factor activity levels differed among the three groups, and a large difference was observed between the upper and lower/concordant groups. In the lower group, it was considered that factors II, VIII, and XI could significantly affect APTT because the levels of these three factors in the group were higher than those in the concordant group.

This study had some limitations. First, this study had a small sample size and was conducted at a single center. Second, clinical outcomes such as thrombotic events and bleeding complications were not evaluated. Third, the coagulation factor VIII activity level in many samples exceeded the upper measurement limit and could not be measured accurately. Unfortunately, due to insufficient sample volume, additional diluted measurements were not possible. Fourth, detailed coagulation characteristics of patients before heparin administration were not demonstrated because of a lack of sample collection for the study. The detailed association between coagulopathy and APTT prolongation in critically ill patients without heparin is unclear.