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I think we should retest D-dimer maybe after a few weeks, three to four weeks. This is to see the trend and confirm the elevated levels. The usual causes for elevation include DVT (deep vein thrombosis), pulmonary embolism, infection, stroke, trauma, and rarely some cancerous conditions. Also, D-dimer rises with age. But, all your investigations are negative so ruling out the major and important causes for elevated D-dimer.
I guess at this moment, you are totally asymptomatic, right? No fever or any evidence of infection, etc. The elevation from the previous infection five months back should have settled by this time. You have undergone all possible tests to rule out the causes of elevated D-dimer, so I think there is no need to worry about it at this moment. So overall, I think we should repeat the test after three to four weeks. Also, see if you have any symptoms, or let me know if you suffered from a recent infection or trauma.