Evaluation of FVIII:C/VWF:Ag Ratios in Children and Adults, with and without VWD

The use of coagulation assays in children is hindered by the lack of well-established, age-appropriate reference ranges. During an ongoing effort to establish these ranges, we noted an elevated ratio of factor VIII activity (FVIII:C) to von Willebrand Factor (VWF:Ag) in healthy children.

We compared ratios among age groups in children, and with adults, in a hypothesis-generating effort toward future elucidation of age-related physiologic mechanisms.

All subjects were enrolled in an IRB-approved study and provided signed informed consent for evaluation. We grouped healthy children by age as follows: 0–5 years (n=25), 6–11 (n=22), and 12–18 (n=14) and compared these groups to our institutional cohort (age 2–64 years, n=47) of type 1 von Willebrand Disease (VWD) patients. We also studied healthy children with voluntary second blood draws (n=13). Finally, healthy adults (n=9) were studied before and after vigorous physical exercise, FVIII:C was measured by one-stage assay and VWF:Ag by ELISA. Statistical analyses employed Mann-Whitney U test to compare distributions of data between groups.

Table 1 shows FVIII:C, vWF:Ag, and ratios by age group. The overall median (observed range) for VIII:C/vWF:Ag ratio for all healthy children is 1.48 (0.73–3.19). This ratio is higher than expected based upon evaluation of commercially-available normal adult plasma (n=48), with a median ratio of 0.95 (0.6–1.52), a value consistent with published data. The range of ratios in healthy children overlapped those in our institutional cohort (n=22) of pediatric patients with type 1 von Willebrand disease (VWD), with an overall median ratio of 1.46 (0.89–3.53). The adult vWD cohort (n=23) had a median ratio of 2.20 (0.54–5.00). Distribution of values for the FVIII:C/VWF:Ag ratio differs significantly between healthy subjects and those with VWD for the following age groups: 0–5 years (p=0.026), 6–11 years (p=0.024), and >18 years (p=<0.001).

Repeat samples in a subset of healthy children (n=13) yielded ratios that were on average slightly higher than on the first draw, largely attributable to an interval increase in FVIII:C. Our evaluation of 9 healthy adult volunteers demonstrated that vigorous exercise led to a 47 percent increase in FVIII:C levels but only 25 percent increase in VWF:Ag levels, resulting in a 16 percent increase in the ratio.

The observation that healthy children had higher FVIII:C/VWF:Ag ratios than their adult counterparts, while adults with VWD had higher ratios than their pediatric counterparts, may be driven by altered VWF stress response or clearance and warrants further study, perhaps including VWF propeptide analysis.

Our findings raise the question of whether FVIII:C/VWF:Ag ratios are physiologically increased in children relative to adults, or whether this is the result of pre-analytic conditions (specifically, an age-related mental stress response to venipuncture). Another possible explanation for the unexpectedly high ratio in healthy children, as supported by our evaluation of exercise response, is a disproportionate elevation in VIII:C with such physical activities as vigorous play in the clinic waiting area, or struggling during venipuncture.

Though the FVIII:C/VWF:Ag ratio differs significantly between healthy subjects and those with VWD in most age groups, the substantial overlap of observed ranges suggest that a ratio threshold value-based screening approach alone cannot reliably discriminate between these groups. The diagnostic performance of this ratio, as measured by ROC curve, is poor for VWD in children, and should not be used as a screening tool in the pediatric population. Further evaluation in larger studies is warranted.

No relevant conflicts of interest to declare.