Is d dimer reliable for excluding DVT pe low risk?

Yes. Yes, a negative d dimer pretty reliable for excluding DVT in a person at low risk for a PE.
D-DIMER. D-dimer is significant when it is negative since it excludes clot formation. A positive d-dimer, however, is not diagnostic of a clot since it can be positive for other reasons. In a low risk patient with a negative d-dimer, the chances of a DVT are very low.
D dimer risk. D-dimer has a high negative predictive value to rule out DVT when risk is low. When risk is high, it is less reliable and ultrasound is better relied upon to exclude DVT.
D-dimer . YES. In rare cases if you have factor XIII deficiency, or the tube was incorrectly filled ,if it is more then 10 days since the event, or use of anticoagulants it may give a false negative but otherwise it is fairly specific for excluding PE/DVT especially if you have not had a previous DVT/PE do not have shortness of breath fast heart rate, calf swelling cancer , recent surgery or travel.