Severe Bleeding Due to an Acquired FXIII Inhibitor in an Otherwise Healthy Patient

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

In this manuscript, Santonastaso and colleagues report a case of severe bleeding associated with acquired factor XIII deficiency. They describe marked bleeding despite FXIII replacement therapy and difficulties in laboratory detection of an inhibitor. While this case provides clinically important insight into the complexity of acquired FXIII deficiency, this reviewer has several concerns regarding the interpretation presented in the Discussion, which require revision as outlined below.

1. Although FXIII concentrate was administered, there is no specific evaluation of half-life. Acquired FXIII deficiency can involve non-neutralizing (clearance-enhancing) antibodies that are difficult to detect by Bethesda assay. The authors’ description of “rapid clearance” strongly suggests this underlying pathology. Given that bleeding symptoms recurred even under corticosteroid therapy, it is reasonable to assume that the antibodies remained present and functional. Therefore, the authors should analyze the decline in FXIII levels after administration after FXIII concentrate and provide a quantitative assessment of the half-life. Demonstrating a significant shortening of the half-life would serve as a crucial objective indicator of an inhibitor.
2. The authors question the commonly accepted concept that FXIII activity levels above 5% are sufficient for hemostasis. However, this threshold has been mainly discussed in the setting of prophylactic management for congenital FXIII deficiency. In acquired FXIII deficiency associated with inhibitors, it is well known that the measured FXIII activity does not always reflect effective in vivo hemostasis. Therefore, the observation that this patient developed severe bleeding despite FXIII activity levels above 5% should not be interpreted as evidence against the established threshold for congenital deficiency, but rather should be discussed in relation to functional inhibition of FXIII by the antibody.

Author Response

1. Although FXIII concentrate was administered, there is no specific evaluation of half-life. Acquired FXIII deficiency can involve non-neutralizing (clearance-enhancing) antibodies that are difficult to detect by Bethesda assay. The authors’ description of “rapid clearance” strongly suggests this underlying pathology. Given that bleeding symptoms recurred even under corticosteroid therapy, it is reasonable to assume that the antibodies remained present and functional. Therefore, the authors should analyze the decline in FXIII levels after administration after FXIII concentrate and provide a quantitative assessment of the half-life. Demonstrating a significant shortening of the half-life would serve as a crucial objective indicator of an inhibitor.

Thank you for your comment. It is appropriate to expand on our discussion regarding what type of inhibitor might be present in this patient and how it ties to the laboratory results we have observed. As recommended, we have adjusted the conclusion section marked in yellow highlight on page 5.

2. The authors question the commonly accepted concept that FXIII activity levels above 5% are sufficient for hemostasis. However, this threshold has been mainly discussed in the setting of prophylactic management for congenital FXIII deficiency. In acquired FXIII deficiency associated with inhibitors, it is well known that the measured FXIII activity does not always reflect effective in vivo hemostasis. Therefore, the observation that this patient developed severe bleeding despite FXIII activity levels above 5% should not be interpreted as evidence against the established threshold for congenital deficiency, but rather should be discussed in relation to functional inhibition of FXIII by the antibody.

The first paragraph of the conclusions section on page 5 was revised to specify congenital deficiency guidelines. Additional information and references were added pertaining to acquired deficiency recommendations and limitations.

 

 

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript addresses acquired FXIII deficiency due to a suspected inhibitor in an otherwise healthy elderly patient. The report provides a detailed description of the clinical course and management strategy. However, several issues should be addressed before it can be considered for publication.

Abstract
The diagnostic uncertainty regarding the FXIII inhibitor should be stated more clearly. While the Abstract implies the presence of an inhibitor, confirmation was not achieved. This limitation should be explicitly acknowledged.

Case
The manuscript states that chromogenic and functional FXIII assays were unable to demonstrate an inhibitor, yet the diagnosis of an acquired FXIII inhibitor is inferred based on clinical behavior and response to immunosuppressive therapy. Please clarify if there are the diagnostic criteria used to support this conclusion and explicitly distinguish between laboratory-confirmed and clinically suspected inhibitors.

Mixing studies, Bethesda assays, or serial inhibitor testing are mentioned in the Discussion but are not clearly described as having been performed or being unavailable in this case. Please clarify whether these tests were attempted and, if not, discuss the reasons for their omission.

Discussion
The Discussion provides a comprehensive overview of FXIII physiology and deficiency but would benefit from a more focused comparison with previously reported cases of acquired FXIII inhibitors. Specifically, please clarify how this case differs in terms of clinical presentation, diagnostic challenges, or response to therapy compared with similar cases in the literature.

One important limitation that should be more explicitly acknowledged is the absence of definitive laboratory confirmation of an FXIII inhibitor. While the clinical course strongly suggests an acquired inhibitor, the lack of confirmatory testing limits diagnostic certainty and generalizability. This should be clearly stated in the Discussion or Conclusions section.

Author Response

1. Abstract
   The diagnostic uncertainty regarding the FXIII inhibitor should be stated more clearly. While the Abstract implies the presence of an inhibitor, confirmation was not achieved. This limitation should be explicitly acknowledged.

A sentence was added to the abstract to reflect the uncertainty of the laboratory testing and how a diagnosis was made despite this.

2. Case
   The manuscript states that chromogenic and functional FXIII assays were unable to demonstrate an inhibitor, yet the diagnosis of an acquired FXIII inhibitor is inferred based on clinical behavior and response to immunosuppressive therapy. Please clarify if there are the diagnostic criteria used to support this conclusion and explicitly distinguish between laboratory-confirmed and clinically suspected inhibitors.

There were no specific diagnostic criteria that was used to diagnose this patient. Due to the rare nature of the disorder and limited case reports published about it, diagnosis is guided by clinical presentation, basic laboratory testing, and potential use of non-FDA approved assays. There is no existing FDA-approved laboratory test to diagnose FXIII inhibitors alone. These details were added to the manuscript on page 2, marked in pink highlight.

3. Case

Mixing studies, Bethesda assays, or serial inhibitor testing are mentioned in the Discussion but are not clearly described as having been performed or being unavailable in this case. Please clarify whether these tests were attempted and, if not, discuss the reasons for their omission.

A modified mixing study was attempted but was not included because it was not marketed as an established “FXIII” inhibitor assay. Page 2 now includes information about the local testing we performed.

4. Discussion
   The Discussion provides a comprehensive overview of FXIII physiology and deficiency but would benefit from a more focused comparison with previously reported cases of acquired FXIII inhibitors. Specifically, please clarify how this case differs in terms of clinical presentation, diagnostic challenges, or response to therapy compared with similar cases in the literature.

Please see page 5 for edits pertaining to the novelty of this case report versus the others already published. The severe bleeding diathesis and other clinical evidence demonstrated the need to start immunosuppressive therapy as soon as possible. Laboratory confirmation was not a requirement prior to this decision because of the severity of bleeding and the lack of an FDA-approved assay.

5. Discussion

One important limitation that should be more explicitly acknowledged is the absence of definitive laboratory confirmation of an FXIII inhibitor. While the clinical course strongly suggests an acquired inhibitor, the lack of confirmatory testing limits diagnostic certainty and generalizability. This should be clearly stated in the Discussion or Conclusions section.

Thank you for your comment. The revisions have been made to the conclusion section, which also aligns with reviewer 1’s comments. Changes have been made to page 5 and 6 to better acknowledge the lack of laboratory evidence.

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

Thank you for the revisions.