Here is a follow-up on yesterday’s (July 23, 2019) case study describing chronically elevated D-dimer. Our correspondent has graciously offered her permission to post her message (anonymously, of course).
Another question for you. If there were a blood clot in my lungs would the CT chest scan be a definitive factor for ruling it out? Or is there a back up test that can ensure there isn’t a blood clot? I ask because my PCM referred me to a pulmonologist. It took a whole month just to get that appointment so my PCM sent me to LabCorp to get the CT chest scan done prior to seeing the pulmonologist. I had it done with dye of course. At the end of the scan I chatted with the technician and told him we are looking for blood clots in my lungs. He mentioned that if he had known specifically that it was for a clot he would have done the CT scan differently. I didn’t understand that. He stated that my order for the scan only said chest CT scan with no reason on it. He said the scan he did “should” show if there were clots. That made me feel really uneasy. I wasn’t aware that there were different types of chest CT scans. So I took that scan to the pulmonologist and he used that for my diagnosis of no clot. I was told too many scans isnt safe so I didn’t do another. Should I be concerned that my scan may have missed a clot?
First some superficial observations. From personal experience, a month is a relatively short waiting time for a pulmonology consult, though as the patient, it may seem like an eternity. Your primary’s plan to get a preliminary CT scan was a good idea, although some specific communication may have improved the diagnosis. Given what appears to be competent exams by your primary and specialists, and assuming from your description that your oxygen saturation is normal and you are not short of breath, you need not be anxious about a possible pulmonary embolism. Of course, as a medical laboratory scientist, my opinion does not rise to the level of a medical diagnosis.
Here’s a more in-depth laboratory-based description. As you’ve experienced, physicians follow carefully developed guidelines for PE diagnosis, beginning with a clinical impression based on a history of chest pain and observed shortness of breath with reduced oxygen saturation. The physician then orders the initial tests: a routine chest X-ray and the D-dimer assay. If the chest X-ray shows no clots and the D-dimer result is normal, the PE is ruled out and the physician looks for alternate chest pain causes such as heart attack or pleurisy.
Medical laboratory scientists and radiologists use the terms sensitivity and specificity to describe the qualities of a diagnostic assay. In a perfect world a diagnostic test would always be positive when the suspected condition is present and negative when the condition is absent; however this only happens in our dreams. In reality every diagnostic procedure has a defined false positive (false alarm) rate and a defined false negative (missed diagnosis) rate. This is why physicians judiciously choose a series of diagnostic assays. In the case of pulmonary embolism, the D-dimer is sensitive and the chest X-ray is specific, so taken together, they provide reasonable diagnostic assurance that no embolism is present.
However, in your case, your D-dimer result was elevated. Given the physician’s clinical impression from your history and physical, it is most likely the elevated D-dimer indicates some other condition such as low-grade chronic inflammation. Nevertheless, the elevated D-dimer motivates the doctor to order a follow-up procedure, the CT scan in accordance with international guidelines.
CT scan methodology continuously progresses to higher and higher definition. The CT scan most often used for detecting clots in lungs is the “multidetector” CT scan. which offers high resolution and good sensitivity and specificity rates. We don’t know exactly what level of CT scan was provided by LabCorp except to observe that LabCorp keeps up with emerging technology to maintain its ability to provide accurate diagnoses.
To summarize, although no diagnostic assay is perfectly sensitive and perfectly specific, the combination of D-dimer, chest X-ray, CT scan, and especially the clinical impressions of your primary care physician, hematologist, and pulmonologist altogether provide you with reasonable assurance that no lung clots are present. Meanwhile, all of us should seek medical assistance when we experience chest pain and shortness of breath.
Again, I hope this helps, and I encourage questions and comments. Continue to watch this post, as we may receive additional comments from expert participants.