D-Dimer use strictly for it's high NPV?

[mauricekenter]

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So I was listening to last months EMRAP and they had a D-Dimer rant on there about using the 99% NPV of the D-Dimer test (as long as it is an ELISA) to basically convince you of excluding a PE in those patients in which you were going to get a CT chest PE protocol.

Now I was under the impression that NPV is based upon the prevalence of the disease in the population. Therefore if you already have a high suspicion of PE in your patient wouldn't this make the prevalence of this disease in the specific patient population higher and therefore lower the NPV and make this less applicable? I have always done this method myself, I just want to get things straight in my head.

Any help would be appreciated. Thanks!

 

[keeping-it-real]

I can't comment specifically on the EMRAP lecture, but the current literature I'm aware of states that the d-dimer is not sensitive enough to rule out PE in people who you presume to have a PE.

Maybe EMRAP was able to cite new literature, but any current EM reference would not support use of a d-dimer to "rule out" PE in a person that you highly suspect a PE. Anecdotally, I've personally seen a saddle PE in someone with a "negative" PERC and negative d-dimer.

 

[Rendar5]

I've seen it too early on in residency. septic lady with what I thought was a Low WELL's Score and a negative d-dimer. CC fellow examined her, felt one leg as larger than the other, and CTA's her since he gave her a moderate Well's risk. Segmental PE plain as day on CT.

 

[WilcoWorld]

To the OP's question - NPV most certainly does depend on disease prevalence. I listened to the same EM Rap and the commentator does not report the prevalence of PE in his study population. If you want to properly interpret their results, you'll need to look at that as well.

 

[Groove]

I've seen it too early on in residency. septic lady with what I thought was a Low WELL's Score and a negative d-dimer. CC fellow examined her, felt one leg as larger than the other, and CTA's her since he gave her a moderate Well's risk. Segmental PE plain as day on CT.

How can she have a low Well's with unilateral leg swelling that would make you suspect a DVT? That's already 3 points which puts her in the moderate risk category. Wait a sec... you ordered a d-dimer on a septic pt and expected it to be normal? Man, I would have just spun her and been done with it. The clinical findings alone would have made me skip it and if she was septic, I would have certainly skipped it...spin that chick. That's if you were thinking PE in the first place, which I suppose you were.

Some of those segmental and def sub segmental PE's.... man, am I the only one that can't see anything half the time? I get headaches trying to find them after the read comes back. Then I might comment to a colleague "oh yea...check out this PE...see it?...wonder what the read says..." What he doesn't know is that I looked at it 10 minutes ago and thought to myself "yep...no PE". I like to always blame it on the monitor. Saddle? Close to the main branches? Now, I can see that.

😉

 

[Rendar5]

How can she have a low Well's with unilateral leg swelling that would make you suspect a DVT? That's already 3 points which puts her in the moderate risk category. Wait a sec... you ordered a d-dimer on a septic pt and expected it to be normal? Man, I would have just spun her and been done with it. The clinical findings alone would have made me skip it and if she was septic, I would have certainly skipped it...spin that chick. That's if you were thinking PE in the first place, which I suppose you were.

Some of those segmental and def sub segmental PE's.... man, am I the only one that can't see anything half the time? I get headaches trying to find them after the read comes back. Then I might comment to a colleague "oh yea...check out this PE...see it?...wonder what the read says..." What he doesn't know is that I looked at it 10 minutes ago and thought to myself "yep...no PE". I like to always blame it on the monitor. Saddle? Close to the main branches? Now, I can see that.

😉

I didn't think she had leg swelling and neither did my attending. It was the fellow who saw the patient after the sepsis became apparent that felt she had lower leg swelling.

Thus, she didn't score high on Well's at all on initial evaluation.

 

[TBS12]

Yesterday I had a scary patient that makes me re-think everything I know about working up a PE. Healthy, early 40 year old female with 2 days of left sided back (rib) pain. Pleuritic in nature, pain increases with cough and movement of her left arm and it hurts to lay on. Her pain started a day after doing yard work (she was pulling a big shrub out of her yard). Not tachycardic or tachypnic, 99% on RA. Normal lung sounds, palpation of posterior chest wall reproduces her exact pain. Wells score of 0, perc negative.

Thinking patient was very low risk, just got CXR and EKG. CXR showed small (very small) left sided effusion. Not having any explanation for effusion we got CT chest and she had bilateral PE's.

So, did her PE cause the effusion? Possibly. If she didn't have a small effusion we would have definitely sent her home.

I guess the kicker should have been the fact that she is a radiology tech and she says she does V/Q scans all day.

 

[xaelia]

Yesterday I had a scary patient that makes me re-think everything I know about working up a PE.

Scary? Stable? No hemodynamic compromise? Not hypoxic? Sounds like you found someone whose lungs were performing their physiologic function of clearing out clots before they reached the arterial circulation. I'm sure that six+ months of anticoagulation will be less harmful than the pulmonary emboli, with all the good evidence we have in support of anticoagulation....

Our goal in the Emergency Department is not to catch every disease - but to use our diagnostic testing to decrease the posterior probability of disease to the point at which further testing would generate more false positives than true positives AND the treatment of the disease would cause more harm than the disease itself. Every "great catch" in a completely unexpected clinical situation is far outweighed by harm of all the negative studies plus the side effects of the treatment if you change your practice based on that single case.

CTA had a lot of false positives in the low-risk group of the 2006 PIOPED II study. I'm betting our more detailed scanners are picking up more physiologic clot burden now, with no additional morbidity or mortality benefit from treatment.

And, just back to the OP - NPV is not a useful statistical construct. It's better to use +LR and -LR to describe the utility of a test.

 

[Groove]

Good catch. Cases like that make me have such a low threshold of CT'ing pt's. I like to think I would have gotten a d-dimer and that PE would have floated through my mind, but as you're describing...I just as easily could have dx muscle strain or chest wall syndrome.

I know there is a lot of new literature that medicine is using to justify outpatient tx of PE where you discharge from the ED on lovenox for many of these segmental and sub segmental PE's, low risk pt's, but regardless of the studies, has anyone else found that commonplace in your institution? Call me crazy, but I still can't imagine feeling comfortable sending anyone out from the ED with a dx PE without further workup. I know it's not 100% indicated, but hell... if I had a PE, I'd want bilateral dopplers of my lowers and I'd even want a TTE to show if I had a PFO, not be sent out on lovenox with nothing else done. (let's imagine it's as cheap as coumadin for the sake of argument, and that all our pt's can afford it).

 

[mauricekenter]

I went back and looked at the original study in which the EMRAP Rant was posting about and it turns out the prevalence of PE between everyone who received a CTA in their study was was < 5%. Yet in the original PIOPED study the prevalence was 33%.

Here is the study:

Computed Tomography Angiography in Patients Evaluated for Acute Pulmonary Embolism with Low Serum D-dimer Levels: A Prospective Study

 

[goodoldalky]

Scary? Stable? No hemodynamic compromise? Not hypoxic? Sounds like you found someone whose lungs were performing their physiologic function of clearing out clots before they reached the arterial circulation. I'm sure that six+ months of anticoagulation will be less harmful than the pulmonary emboli, with all the good evidence we have in support of anticoagulation....

I think the above makes a lot of sense. The patient would have likely had a good outcome had she been discharged home without the CTA. I mean, in this case, it seems that you discovered an irrelevant, clinically unimportant PE. That is, however, unless the bilateral PE's are some sort of sentinel event signifying some sort of coagulopathy or larger problem, in which case this workup may help her out down the road.

If you look at the numbers, the advent of CTA has enabled us to diagnose PE's that were previously undiscovered (estimated by this study http://www.ncbi.nlm.nih.gov/pubmed/21555660 as 50 cases per 100,000 that were previously missed).

The benefit was that PE mortality decreased from 12.3% to 11.9% (statistically significant), but that complications of anticoagulation - GI Bleed, ICH, HIT, increased from 3.1 to 5.3/100,000 (again, statistically significant). My math is fuzzy but I do believe that these numbers drawn out mean that you save 1 episode of PE mortality for each group of 50 additional diagnoses, but that your complications of anticoagulation are increasing ~70%. Another cited study in this article reveals that 12% of patients anticoagulated for 3-6 months experience clinically significant bleeding, meaning that 6 of those 50 additional people you diagnosed with PE will experience harm due to the treatment (that does not include harm related to IVC filter placement, radiation from CT scans, contrast nephropathy, etc).

I'm certainly no expert in statistics, but it seems to me that in the United States (for many reasons, i.e.: CYA), we overdiagnose many otherwise clinically irrelevant PE's and that's not necessarily to the benefit of patients.