Stains & CD markers
Factor XIIIa

Last author update: 20 April 2022
Last staff update: 20 April 2022

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PubMed Search: Factor XIIIa[title] stain

Valeriya Skorobogatko, B.A.
Brandon Umphress, M.D.
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Cite this page: Skorobogatko V, Umphress B. Factor XIIIa. website. Accessed February 23rd, 2024.
Definition / general
  • FXIIIa is widely expressed in a variety of cell types; however, it is most commonly recognized as a marker of fibrohistiocytic proliferations
  • Derived from monocyte / macrophage myeloid lineage and expressed in dermal dendrocytes, primarily surrounding microvasculature in the adventitial dermis near the dermoepidermal junction and near skin appendages (Int J Mol Med 2008;22:403)
  • Active form of factor XIII, the last enzyme in the coagulation cascade
Essential features
  • Cytoplasmic marker with expression in normal epithelial tissue and various pathologic states
  • Mainly used to distinguish dermatofibroma from dermatofibrosarcoma protuberans
  • Generally expressed (with some variation) in benign sebaceous neoplasms, fibroblasts in fibrovascular tumors and a proportion of histiocytomas
  • Generally negative (with some variation) in malignant sebaceous neoplasms and clear cell tumors
  • Factor XIIIa, FXIIIa, F13a, fibrin stabilizing factor
Uses by pathologists
  • Marker of fibrohistiocytic proliferations
  • Marker of dermal dendrocytes
  • Differentiates dermatofibrosarcoma protuberans (CD34+, factor XIIIa-) from dermatofibroma or benign fibrous histiocytoma (FH) (CD34-, factor XIIIa+)
  • FXIIIa (AC-1A1) is a sensitive and specific marker for sebaceous differentiation (J Cutan Pathol 2018;45:1)
  • Shown to be expressed in higher numbers in indeterminate leprosy compared to normal skin (Am J Dermatopathol 2015;37:269)
Prognostic factors
  • FXIII has been shown to have a role in metastatic potential by impeding NK cell mediated clearance of tumor cells (J Thromb Haemost 2008;6:812)
  • Lung squamous cell carcinoma: plays a role in cell invasion and disease progression; extensive fibrin linkage correlated with worse prognosis (Nat Commun 2018;9:1988)
  • Low levels of FXIIIa following an acute myocardial infarction have been correlated with worse prognosis with increased complications, such as heart failure and death (Thromb Haemost 2015;114:123)
  • Cytoplasmic staining ranging from focal to diffuse and weak to strong
Microscopic (histologic) images

Contributed by Brandon Umphress, M.D.

Dermatofibroma, FXIIIa positivity

Dermatofibroma, diffuse FXIIIa positivity

Fibrohistiocytic proliferation, partial FXIIIa+

Cellular fibrous histiocytoma, FXIIIa+

Benign fibrous histiocytoma, FXIIIa+

Positive staining - normal
Positive staining - not malignant
Positive staining - malignant
Negative staining - disease
Sample pathology report
  • Skin, thigh lesion, biopsy:
    • Dermatofibroma (see comment)
    • Comment: Immunohistochemical studies with FXIIIa demonstrate diffuse positivity within tumor cells, while CD34 is negative. The findings are supportive of the diagnosis of dermatofibroma.
  • Skin, back mass, excision:
    • Dermatofibrosarcoma protuberans, margins free (see comment)
    • Comment: Immunohistochemical studies with CD34 demonstrate diffuse positivity within the tumor, while FXIIIa is negative. The immunohistochemical findings are in support of the diagnosis.
Board review style question #1
A 37 year old woman presents with a 1 year history of a lumpy mass on her midback. On physical exam, the lesion is 3 cm, slightly raised, resembling a scar that feels rubbery upon palpation. Histologic examination shows a spindle cell neoplasm that is diffusely positive for CD34 and negative for factor XIIIa and S100 staining. Which of the following is the most likely diagnosis?

  1. Cutaneous melanoma
  2. Dermatofibroma
  3. Dermatofibrosarcoma protuberans
  4. Myxoid liposarcoma
Board review style answer #1
C. Dermatofibrosarcoma protuberans is CD34+, factor XIIIa- and S100-. Dermatofibroma is typically CD34- and factor XIIIa+. Myxoid liposarcoma is usually vimentin+, S100+ and CD34-. Cutaneous melanoma would express S100.

Comment Here

Reference: Factor XIIIa
Board review style question #2

Which of the following features regarding factor XIIIa is true?

  1. Commonly differentiates melanoma from basal cell carcinoma
  2. Marker of fibrohistiocytic proliferations
  3. Part of the coagulation cascade aiding in plasminogen crosslinking
  4. Stains negatively for normal dermal dendrocytes
Board review style answer #2
B. Factor XIIIa is a general marker for fibrohistiocytic proliferations

Comment Here

Reference: Factor XIIIa
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