When Blood Clots Too Easily: The Genetics Behind Inherited Thrombophilia

Blood clotting is a balancing act. Clot too little and a minor injury becomes an emergency. Clot too much and the same machinery starts blocking healthy vessels — in the deep veins of the leg, in the lungs, sometimes in the placenta. For most people that balance holds. For a significant minority, it is tilted from birth by a handful of inherited variants they will never know about until something goes wrong.

Venous thromboembolism is among the most common cardiovascular conditions worldwide, and many first episodes occur without an obvious trigger. When a clot appears in a young patient, recurs without explanation, or runs in a family, the question stops being what happened and becomes why this person.

What routine coagulation testing misses

Acquired risk factors — surgery, immobilisation, malignancy, pregnancy, oestrogen-containing contraceptives, are usually visible in the history. Inherited thrombophilia is not. It is present from conception, silent for decades, and invisible to routine screening: a normal PT and aPTT say nothing about it. Most of that inherited risk traces back to a small number of well-characterised variants.

Factor V Leiden (G1691A). Activated protein C normally inactivates Factor Va and shuts the cascade down. This single base change alters the cleavage site, leaving a Factor Va that resists inactivation — activated protein C resistance. It is the most common inherited thrombophilia in European populations (roughly 3–8%), rarer in South Asian, East Asian and African populations. Risk rises sharply when it meets an acquired trigger such as contraception, pregnancy or orthopaedic surgery.

Prothrombin G20210A (Factor II). This variant sits in the 3' untranslated region and doesn't change the protein at all. It makes mRNA processing more efficient, leaving carriers with elevated circulating prothrombin — more substrate for thrombin generation. Found in about 1–2% of European populations. Its real significance is combinatorial: a patient carrying both Factor V Leiden and G20210A faces considerably higher risk than either variant alone, which is why testing one marker in isolation can mislead.

MTHFR (C677T and A1298C). These variants reduce enzyme activity in folate metabolism and can raise plasma homocysteine. Worth being straight about: major professional bodies now advise against routine MTHFR genotyping in a standard thrombophilia workup, since genotype alone predicts thrombotic events poorly. Its value sits in a narrower frame — evaluating folate–homocysteine metabolism, interpreting a documented raised homocysteine, or building a fuller metabolic picture in reproductive medicine. Reported alongside Factor V and Factor II with that context attached, it informs; reported alone as a clot-risk verdict, it doesn't.

Who to test, and why as a panel

Genotyping earns its place when the result could change management: unprovoked VTE at a young age, recurrent events, thrombosis at unusual sites, strong family history, recurrent pregnancy loss under investigation, or counselling at-risk relatives before contraception or pregnancy. DNA-based testing has a practical edge over functional assays here — the genotype doesn't shift with acute thrombosis, anticoagulation or sampling time.

Running the three markers separately means three aliquots, three runs, three sets of controls and an answer arriving in instalments. Since the combinations carry different clinical weight than the individual variants, the complete genotype is the actual deliverable.

TRUPCR® Coagulation/Thrombophilia Panel qPCR Kit

The kit detects Factor II (Prothrombin G20210A), Factor V Leiden (G1691A) and MTHFR (C677T and A1298C) from a single sample in one run. Allelic discrimination chemistry gives clean, reproducible genotype calls; sensitivity reaches 2% mutant allele in a 98% wild-type background; and wild-type amplification in every reaction acts as a built-in check on DNA quality, so a failed extraction declares itself instead of passing as a negative.

References:

• Cripe LD, Moore KD, Kane WH. Structure of the gene for human coagulation factor V. Biochemistry 1992; 31: 3777-3785.
•  Junker R, Nowak-Göötl U, Fobker M. The methylenetetrahydrofolate reductase (MTHFR) C677T polymorphism and other genetic causes of hyperhomocysteinemia in venous thrombosis. J Lab Med. 2001; 25 (7/8): 239 – 253
• Zhang S, Taylor AK, et al. Venous thromboembolism laboratory testing (factor V Leiden and factor II c.*97G>A), 2025 revision: a technical standard of the American College of Medical Genetics and Genomics (ACMG). Genet Med. 2025; (in press)
• Metz TD, Silverman NS. ACOG Practice Bulletin No. 197: Inherited Thrombophilias in Pregnancy. Obstet Gynecol. 2018; 132 (1): e18 – e34