Here is a follow-up comment from Dr. Larry Brace, Edward Hospital, Naperville, IL.
With regards to the patient with pubertal menorrhagia, has anyone tested the parents? If this is an inherited X deficiency, both parents would need to be heterozygous in order to have a child that has 1% factor X. If both parents have normal X activity, then it must be assumed that the patient has an acquired X deficiency, or that the patient is adopted. There is a fairly long list of conditions that cause acquired X deficiency, but most would not be suspected in such a young patient.
In addition, I would also suggest testing factor X by a chromogenic X assay. This would ameliorate the issue involved in different PT and PTT reagents used in most X activity assays. The patient should also be tested for factor X antigen.
Here is an additional comment from Vilas Hiremath on the same issue:
Dear George, Why is PTT normal in factor X deficiency? Because PT is more sensitive than PTT for the detection of vitamin K-dependent coagulation factors like factor X.
Response from George:
While it doesn’t answer Vilas’ question about why only the PT is affected, I like Dr. Brace’s suggestions, which can be used to confirm factor X deficiency. The unusual PT and PTT results make it necessary to reach a conclusion. I’m not sure that I agree with Vilas’ assertion that the PT is more sensitive than the PTT for factor X, I believe both are equally sensitive, within the limits of their developers’ specifications. Nevertheless, the issue raises uncertainties that will hopefully draw further discussion.