drabtshirt

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I have heard from several residents that d-dimer is not accurate in patients who have been in the hospital for longer than a few days, but not sure why.

Is this true? Do you know why?
 

Varmit22

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I have heard from several residents that d-dimer is not accurate in patients who have been in the hospital for longer than a few days, but not sure why.

Is this true? Do you know why?



D-dimer has great predictive value if negative. But a positive d-dimer, no matter in what setting does not mean much. Positive predictive value is ~30%, and negative predictive value is >90%.
D-dimer is a product of clot lysis, so in patients who have been inactive in the hospital for a period of time, they may have an elevated d-dimer due to venous stasis and small clot formation, which probaly is not clinically significant. Patients also undergo procedures that cause clot formation, like IV placement, etc. D-dimer can also be elevated in many other conditions, such as CHF, renal failure, advanced age, pregnancy, sepsis, recent surgery, medications, and malignancy. Hospitalized patients have many of these features already. Hope this helps.
 

Emedpa

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also a clot present for > 1 week may have a neg d-dimer.
it is a marker of acute clot formation.
I have seeen several fairly large pe's and dvt's with neg d-dimer assays in folks with sx over 1 week.
apparently there are several assays out there and the elisa is a better assay than the non-elisa.
 
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HomerSD

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The d-dimer test is only useful in the context of the clinician's pre-test probability of a venous thromboembolic event. An ELISA <500 is typically useful to exclude DVT or PE, but only if the pre-test suspicion of PE/DVT is low or moderate. In patients with a high pre-test probability, there is very little utility in checking a d-dimer and I would proceed directly to imaging.

The d-dimer has also been documented to be elevated in hospitalized patients, particularly those with a malignancy or recent surgery. I typically don't check d-dimers in inpatients, as the results can be difficult to interpret.

PIOPED recommendations (It may be restricted to subscribing institutions, sorry)
 

bulgethetwine

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also a clot present for > 1 week may have a neg d-dimer.
it is a marker of acute clot formation.
I have seeen several fairly large pe's and dvt's with neg d-dimer assays in folks with sx over 1 week.
apparently there are several assays out there and the elisa is a better assay than the non-elisa.

If you've got a pt with symptoms, it is unlikely they are sufficiently low risk to warrant a d-dimer in the first place. Reference the Pioped data.

Also, remember that a d-dimer is likely to be elevated in nearly any condition that has an acute inflammatory or acute inflammatory-like condition: infection, inflammation, pregnancy, etc.

Bottom line: In a patient in which DVT/PE is in the differential, consider the d-dimer test, but use Well's criteria to stratify your patient into "zero", "low", "moderate", or "high" risk. If your patient is virtually zero or low risk, the d-dimer has utility NOW and should be ordered. If high risk, skip the d-dimer and go to imaging. If moderate, the d-dimer MAY have utility after the fact if the imaging study is inconclusive (such as in the case of an indeterminate VQ scan) but most would say that even in this case, the d-dimer is of inferior utility to, say, getting another study such as CT PE protocol or bilateral lower limb ultrasounds.
 

Emedpa

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I find wells crieteria fairly worthless, too ambigous. anyone you are considering seriously has at least a mod prob due to the last criteria( + 3 if PE is more likely than another dx).
I like the perc criteria better as it uses solid #s.
the problem with both is a pregnant female smoker with a pulse of 105 and a cough with pleuritic chest pain who has asthma and this may or may not be different than a typical exacxerbation.....
 

bulgethetwine

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I find wells crieteria fairly worthless, too ambigous. anyone you are considering seriously has at least a mod prob due to the last criteria( + 3 if PE is more likely than another dx).
I like the perc criteria better as it uses solid #s.
the problem with both is a pregnant female smoker with a pulse of 105 and a cough with pleuritic chest pain who has asthma and this may or may not be different than a typical exacxerbation.....

I hear ya. I like the PERC, too... but it isn't validated in as robust a population as the Wells. Besides, PERC or Wells, anyone you are considering "seriously" isn't the patient to get a d-Dimer anyway... that patient should be getting a more definitive test. Your mythical patient above, for instance, would get a V/Q in my ER, I wouldn't even order the damn d-dimer. Plus, someone who is pregnant is gonna have a positive d-Dimer no matter WHAT the symptoms are. The Wells criteria -- and the PERC = FFP (far from perfect). But even though they're imperfect tools, they've saved many a CT scan in my practice when my gestalt KNOWS they don't have a PE, but I need something more to reference (than my gestalt) in the decision-making context of my documentation.

It's all a game, my friends :)
 

Emedpa

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I hear ya. I like the PERC, too... but it isn't validated in as robust a population as the Wells. Besides, PERC or Wells, anyone you are considering "seriously" isn't the patient to get a d-Dimer anyway... that patient should be getting a more definitive test. Your mythical patient above, for instance, would get a V/Q in my ER, I wouldn't even order the damn d-dimer. Plus, someone who is pregnant is gonna have a positive d-Dimer no matter WHAT the symptoms are. The Wells criteria -- and the PERC = FFP (far from perfect). But even though they're imperfect tools, they've saved many a CT scan in my practice when my gestalt KNOWS they don't have a PE, but I need something more to reference (than my gestalt) in the decision-making context of my documentation.

It's all a game, my friends :)

agree- I hardly ever get d-dimers as they have too many false +'s.

our rads like ct > vq for pregnant pts as they say the max radiation dose ends up in the bladder right next to the fetus with vq during excretion
 

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Bottom line: In a patient in which DVT/PE is in the differential, consider the d-dimer test, but use Well's criteria to stratify your patient into "zero", "low", "moderate", or "high" risk. If your patient is virtually zero or low risk, the d-dimer has utility NOW and should be ordered. If high risk, skip the d-dimer and go to imaging. If moderate, the d-dimer MAY have utility after the fact if the imaging study is inconclusive (such as in the case of an indeterminate VQ scan) but most would say that even in this case, the d-dimer is of inferior utility to, say, getting another study such as CT PE protocol or bilateral lower limb ultrasounds.

We had so many problems with residents ordering d-dimers that we had to implement control measures in our CPOE system. If a Wells score is >2, then a resident cannot order a d-dimer. Despite this, residents still find ways to order them on inappropriate patients. This is ironic since I am at one of the six institutions where the PIOPED data originated.

One of the things that is never mentioned with d-dimer sensitivity and specificity is the type of test used. There are variations, and some are better than others. We are currently using a quantitative test, and despite there being evidence to support a certain level as a cutoff, our lab medical director has chosen to lower this level. There is no evidence to support the lower level, and we've found an increase in patients requiring CTA's or V/Q scans with a subsequent decrease in the percent of those found to have PE's.

Yet another mistake in medicine where we think level A is supported by evidence, so we should use a cutoff of level (A - 1) to ensure we never miss anybody. Unfortunately it causes unnecessary radiation exposure from CTA's and V/Q scans.
 

roja

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Key point about d-dimer and relying on PPV/NPV. PPD and NPV are population dependent.

This is why your pretest probability is crucial. I have my residents use Well's for screening. (I actually once had a resident order a ddimer on a patient who had signed out of another hospital the same day with a known DVT...... it was negative, illustrating that ddimers are not 100% and should be used in the RIGHT patients only).

I once has an ICU private attending tell me to order a DDimer on a patient with active cancer, on chemo, with a hx of PE who was tachypnic, tachycardic and hypoxic "because Ddimer has a great NPV.".

Once the pvt attending left, I ordered a CT angio and used this as a teaching example of how a negative predictive value in a HIGH prevalance patient population is meaningless.
 

DeLaughterDO

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I once has an ICU private attending tell me to order a DDimer on a patient with active cancer, on chemo, with a hx of PE who was tachypnic, tachycardic and hypoxic "because Ddimer has a great NPV.".

Once the pvt attending left, I ordered a CT angio and used this as a teaching example of how a negative predictive value in a HIGH prevalance patient population is meaningless.

:thumbup: :smuggrin:
 

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I have a pt with low clinical prob (well's of 0, unlikely) but presents with CP and SOB. A CXR demonstrated an inflitrate c/w PNA. I thought that this was an ok setting in which to order a D-dimer since the pt did have SOB and CP and PE should be considered part of the differential although unlikely. The pt does not have a h/o malignancy or recent surgery, however she is elderly. I ordered the D-dimer and it was positive. What obligation do I have to continue with further testing imaging? I had ordered the D-dimer on admission and by the following day the pt had improved on IV abx. Her dx almost certainly PNA. I ordered the D-dimer b/c if it was neg it would have been very useful but a positive result is near useless due to notorious lack of specificity of the test. As a medical student in Montreal I observed that there was little obligation to cont with further testing. As a resident in the US however I've found that people really try to avoid ordering D-dimers in low prob pts (which is when you are supposed to order them) b/c then if it is positive there is an obligation to cont to test even if the pt is improving. My thought is that if the pt improves w/ Rx for the more likely dx a positive D-dimer in doesn't mean much but if they don't then it is something to keep in the back of one's differential... what are people's thought on this? I want to keep ordering D-dimer's in my low prob pts but I don't want to feel like I have to follow every positive up with a scan.
 

emergiQ

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I have a pt with low clinical prob (well's of 0, unlikely) but presents with CP and SOB. A CXR demonstrated an inflitrate c/w PNA. I thought that this was an ok setting in which to order a D-dimer since the pt did have SOB and CP and PE should be considered part of the differential although unlikely. The pt does not have a h/o malignancy or recent surgery, however she is elderly. I ordered the D-dimer and it was positive. What obligation do I have to continue with further testing imaging? I had ordered the D-dimer on admission and by the following day the pt had improved on IV abx. Her dx almost certainly PNA. I ordered the D-dimer b/c if it was neg it would have been very useful but a positive result is near useless due to notorious lack of specificity of the test. As a medical student in Montreal I observed that there was little obligation to cont with further testing. As a resident in the US however I've found that people really try to avoid ordering D-dimers in low prob pts (which is when you are supposed to order them) b/c then if it is positive there is an obligation to cont to test even if the pt is improving. My thought is that if the pt improves w/ Rx for the more likely dx a positive D-dimer in doesn't mean much but if they don't then it is something to keep in the back of one's differential... what are people's thought on this? I want to keep ordering D-dimer's in my low prob pts but I don't want to feel like I have to follow every positive up with a scan.

The utility of d-dimer when applied to PE is predicated on pre-test probability. But the problem with your case is that now you're trying to decide what to do with post-test reasoning -- in part because apparently your d-dimer came back after a long delay such that you were able to ascertain the effects of antibiotics, resulting in improvement. Where I work (and everywhere I've worked) the d-dimer comes back faster than a time allowing us to assess antibiotic efficacy.

Order it or don't, but if your probability is low, and it comes back positive, my feeling is that you should pursue it with the next test in the algorithm; But I'm equally hesitant to have hard and fast "rules". (EmergiQ clearly straddles fence here :)

In truth, 9.5 times outta 10, i'd just do the next study. But if, for whatever reason, the patient just improves so quickly that you don't think it could possibly be a PE , then document everything and you could still be exonerated from having to do a VQ or CT PEA or lower extremity u/s.

Conclusion on your case as stated though?

I'd just CT PE the patient. Wouldn't hurt to characterize the pneumonia with the CT anyway.
 
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