D-Dimers

Using D-dimer as an adjunct in differential diagnosis of a patient presenting with a possible PE is not a crazy thought in the absence of other conditions that could also cause an elevated result such as recent surgery, trauma, liver disease. The test is part of the algorithm for PE. In your patient's case, the D dimer will be elevated because of the recent surgery and will have limited utility.

The definitive test is a contrast CT of the chest but there are patients who have contraindications to IV dye or who are at risk for contrast nephropathy. Those patients tend to not get scanned so other findings such as elevated D-dimer, lower extremity VTE, and clinical suspicion of PE based on presentation justifies treatment with anticoagulation.

Pneumatic boots can prevent lower extremity VTE but not always. Adding Heparin SQ or Enoxaparin SQ can decrease the likelihood. Ultrasound does not diagnose a PE.

...and there are case reports of patients who was found to have a PE yet never evidence of a lower extremity VTE. The thought is that the clot may have embolized from an upper extremity source or a "de novo" PE (meaning it originated in the lung itself).

D-dimers still have some use, especially in ambulatory settings. I use serial elisa d-dimers in low-risk patients to rule out a DVT while keeping them away from the hospital. I will also use them in younger patients to avoid having to spiral CT them if I am not overtly concerned by their presentation.

How many days post-op would I expect to see an elevated D-dimer in the absence of liver diseases?

There have been various studies that have addressed this. Most of the studies have shown that d dimer is elevated for at least a month post-op with the duration of elevation varying depending on the type of surgery done.

A study done in General Surgery (intra-abdominal and retroperitoneal/liver) showed that d dimer is elevated for 25 days in intra-abdominal surgery and 38 days after retroperitoneal and liver surgery. See: Kinetics of D-dimer after general ... [blood Coagul Fibrinolysis. 2009] - PubMed - NCBI

In Orthopedic Surgery, d dimers were found to be elevated for 30 days after Total Knee Replacement. See: Course of D-dimer concentrations after total knee replacement surgery: effect of allogeneic and unwashed drainage blood transfusion - MU[]OZ - 2006 - Transfusion Alternatives in Transfusion Medicine - Wiley Online Library.

Since this is an ICU forum, realize that in our patient population, there is limited utility of d dimers because our patients have other conditions that cause an elevated d dimer, thus, the predictive value of this test in ruling out a clot is not extensively used by critical care providers.