Thread by @RajeevJayadevan

D Dimer: A test that leads nowhere?

25% of 150 recovered COVID-19 patients had elevated D Dimer at 4 months. 81 were outpatients.

The exact role of D-dimer in COVID-19 is unclear; it does NOT predict PE unless there are concomitant clinical clues.

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healio.com/news/hematolog…

Traditionally D-dimer is elevated when there are fibrin degradation products associated with clots. But D dimer is also elevated in other inflammatory conditions, trauma, post-op state / malignancy.

In COVID-19, its role is primarily as a marker of severity & inflammation.
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Studies in COVID-19 patients have shown that unless there are specific clinical pointers to a clot (leg or lung), an elevated D-dimer value ALONE will not be worth pursuing: we might end up chasing a bird that was never there.

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This study proves exactly that. In many COVID-19 patients, D-dimer is simply a marker of the ongoing inflammation, rather than the signal of a "hidden clot somewhere".

For that reason, it is unwise to depend on an isolated D-dimer value alone to decide on anticoagulation.

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It is also observed that among patients who are admitted to hospital with COVID-19, an elevated D dimer at admission has predictive or prognostic value.

The obvious question comes up now:

How good is the link between PE (pulmonary embolism) and D dimer?

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The relationship is interesting, and invokes the concept of PPV (positive predictive value). In someone with high pretest probability of PE, a high D dimer will help in the diagnosis.

This is called PPV. That is the ability of a positive test to predict the disease.

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In other words, in this context PPV means, out of a 100 positive D dimer tests, how many had PE?

PPV is different from sensitivity - ability of a test to turn positive when done on a diseased person. That is, out of 100 people with PE, how many can the Ddimer test pick up?

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PPV is not fixed - for any test. It varies depending on the clinical setting.

It is like saying, if you hear someone knocking on your door at 9 in the morning, it is likely to be someone you need to meet. But if the same knock occurs at 3 AM, it probably means trouble.

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Likewise, the same test could mean different things depending on the context it was ordered in. In summary, a high D dimer value alone may not mean anything in a COVID patient who is otherwise doing well.

Often, it is good to leave well enough alone.

Linking my article.

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Knowledge of biostatistics is essential in healthcare.

Attaching my article from 3 years ago, written in the context of the tread mill test (TMT) & heart disease, but as an easy teaching example of interpreting the fundamentals of ordering tests.

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onmanorama.com/lifestyle/heal…