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ddimer

D-Dimer pathology test

see also:

  • a raised D-Dimer test does NOT mean there is a clot - there are many other causes of a raised D-Dimer - even just the normal inflammatory response following a vaccination or a bruise!

the D-Dimer test

  • the D-Dimer test was originally designed to detect DIC as D-Dimers are plasmin fibrinolytic breakdown products òf the fibrin mesh that has been stabilised by Factor XIII.
    • generation of D-dimer requires the sequential activity of 3 enzymes:
      • thrombin
        • thrombin generated by the coagulation system converts soluble fibrinogen to fibrin monomers
        • each fibrinogen molecule is a symmetrical dimer composed of 3 pairs of 3 intertwined polypeptide chains, termed α, ß, and γ, that extend laterally from a central core
      • activated factor XIII (factor XIIIa)
        • fibrin strength is enhanced by factor XIIIa, a transglutaminase activated by thrombin, which cross-links the D domains of adjacent fibrin monomers, as well as the α-chains of opposing monomers to form D-dimer and α-polymers, respectively
      • plasmin
        • plasmin degrades the fibrin network when when fibrin-bound plasminogen is converted to plasmin by tissue plasminogen activator
        • plasminogen circulates in plasma, tissue plasminogen activator is released locally from endothelial cells in response to injury
        • plasmin formation is localized to fibrin, ensuring that degradation of circulating fibrinogen is minimal
        • fibrin-bound plasmin degrades the fibrin network into soluble fragments, the building block of which is (DD)E: a complex consisting of D-dimer generated from cross-linked adjacent D domains (DD) noncovalently bound to fragment E
        • further plasmin-mediated proteolysis of fragment E releases it from the (DD)E complex, and D-dimer then circulates in plasma with a half-life of approximately 8 h until it is cleared by the kidneys and the reticuloendothelial system 1)
  • localised fibrin production is an essential component of clot formation for haemostasis and also for localisation of infections, it is thus also formed in many inflammatory states, and also integral in tissue repair after injury, and thus D-Dimer levels are likely to be raised in these scenarios
  • the test was found to be of use in detecting thromboembolism.

causes of raised D-Dimer levels:

upper limit of normal for D-Dimer

  • age < 50yrs: 0.5µg/mL
  • age > 50yrs: age in yrs/100 µg/mL

utility in pregnant patients

  • although there are D-Dimer cutoffs adjusted for trimesters, these have not been adequately validated and thus obstetricians generally DO NOT USE D-Dimer in pregnancy
  • consider discussing all suspected cases of PE with an obstetric service
  • 1st trimester pregnancy: 0.95µg/mL
  • 2nd trimester pregnancy: 1.29µg/mL
  • 3rd trimester pregnancy: 1.7µg/mL7)
  • Kline's approach
    • 1st trimester pregnancy: modified PERC with HR > 105 and D-Dimer > 50% higher than normal cutoff
    • 2nd trimester pregnancy: modified PERC with HR > 105 and D-Dimer > 100% higher than normal cutoff
    • 3rd trimester pregnancy: modified PERC with HR > 105 and D-Dimer > 125% higher than normal cutoff
  • BUT The DiPEP study8) looked at biomarkers and VTE in pregnant women and concluded that D-dimer among other biomarkers were not reliable to rule VTE in or out in pregnancy
  • THUS some suggest the following approach9):
    • IF your gestalt for PE is high risk then do CXR and if normal, a V/Q scan
    • IF gestalt for PE is moderate risk then:
      • if 1st trimester can do D-Dimer and if negative then no imaging
      • otherwise, do CXR to exclude other causes and bilateral leg doppler USS
        • if DVT found then treat as per PE
        • if no DVT then PE is unlikely

false negative D-Dimer results in VTE

  • clot burden is too small
  • early testing
  • delayed testing - levels fall to 25% of initial levels after 1-2 weeks
  • anticoagulant Rx - reduced D-Dimer levels by 25% within 24hrs of Rx

Simply-RED D-Dimer test:

  • whole blood agglutination method
  • sensitivity 66%, NPV 89%
  • if low pre-test probability of PE then NPV almost 100%

Mx of the patient with a raised D-Dimer

1. Is there a reasonable explanation for it?

  • for example: pregnancy, recent surgery, recent trauma / bruise / fractures, severe sepsis, recent vaccination, or known chronic raised DDimer due to malignancy
  • a significantly raised CRP is likely to suggest that a raised D-Dimer is a false positive
  • in general, D-Dimer should not be performed on these patients unless looking for very high levels for another cause such as snakebite or AZ vaccine thrombosis with thrombocytopenia syndrome (TTS / VITT / VIPIT) and if not doing this, the dDimer should probably be ignored and the patient treated on their merits as to their underlying presentation thus EXIT this algorithm.

2. Is the level greater than 5x upper limit of normal?

  • this raises the possibility of:
  • although in cases of high pre-test probability for PE, PE is 4x more likely with D-Dimers > 4ug/mL compared to 0.5-1.0ug/mL, such high levels also raise the possibility of the above conditions thus it is becomes less likely to just be the usual VTE such as PE/DVT without the above conditions without clinical evidence of a massive PE or extensive DVT and when pre-test probabilities of PE are lower
    • this is a bit like having a CRP > 100 with no RIF tenderness when considering appendicitis, whilst it may be appendicitis, other causes should be considered - thus if the clinical picture does not strongly support a large DVT/PE then a very high D-Dimer suggests there may be another cause
    • this is especially the case if the D-Dimer is > 8 ug/mL

3. Abnormally high levels adjusted for age but less than 5x the upper limit of normal

  • this may be a normal post-vaccination inflammatory response!
  • if there is associated thrombocytopenia in the 4-30 days post-AZ vaccine, consider thrombosis with thrombocytopenia syndrome (TTS / VITT / VIPIT) as a possibility
  • If there are symptoms or signs of PE (eg. SOB, chest pain, unexplained persistent tachycardia, near-syncope) then consider CTPA or VQ scan
  • If there are no features of PE but possible symptoms/signs of DVT then consider a Doppler USS legs
  • If there are features of aortic dissection consider a CT aortogram
  • If the presentation is consistent with ruptured abdominal aortic aneurysm (AAA) then investigate urgently with CT aortogram
  • If the patient is on anticoagulants consider the possibility of occult bleeding such as retroperitoneal haemorrhage if there are other features such as pain to suggest this.
  • If none of the above are present:
    • if there are risk factors for aortic aneurysm such as age > 60, male, smoker, raised lipids or FH of AAA, then consider an outpatient screening abdominal USS assuming patient's presentation is stable and not of a ruptured AAA in which case an emergent CT aortogram should be considered

clinical utility in possible VTE:

  • as a method of excluding venous thromboembolism with a normal result (ie. no need to perform V/Q scan or CTPA if normal):
    • ONLY perform if:
      • low pre-test probability of venous thromboembolism:
        • patient < 50yrs age (unless you are happy to use the age-related cutoffs - see above)
        • heart rate : systolic BP ratio < 1
        • no unexplained hypoxia (ie. SaO2 > 95% on room air)
        • no haemoptysis
        • no calf swelling
        • no recent general aneasthetic past 4wks
      • AND other causes of raised levels unlikely:
        • ie. not for most inpatients & pts with substantial comorbidities
      • THUS only for previously well pts presenting to ED with new possible venous thromboembolism but low pretest probability of it

newer ELISA & whole blood agglutination:

  • not available in all hospitals
  • positive if > 500ng/ml
  • high negative predictive values (90%)
  • positive predictive value 30%

immunochromatographic bedside D-Dimer tests:

  • eg. Simplify by Agen Biochemical.
  • similar design to urine pregnancy test strips, results available in 10min
  • in the population whose Hamilton Score for DVT is “unlikely”, then the negative predictive value for an acute lower limb DVT is 98.8%.
  • unfortunately, in the group whose Hamilton Score is “likely DVT” the negative predictive value is only 82% and cannot be used to rule it out and thus USS is indicated.
ddimer.txt · Last modified: 2022/06/18 23:22 by gary1

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