Yes
This may partly depend on your local carrier, so be sure to check that site carefully.
But, in general, yes you may give full credit for the history as long as the reason why it is unobtainable is clearly documented. Example: Patient arrives via EMT, unresponsive due to massive head trauma. No family present. No history other than what was provided by EMT regarding the immediate trauma is available.
Note that in my example I indicate that not only the patient was unresponsive, but no one else was available to provide any history. The EMTs will have some basic HPI info which the physician should record in his/her own note stating something along the lines of "per EMT patient in head-on collision at approx 16.20 hrs today; unresponsive in the field with GCS 5; multiple trauma injuries visible, including ...." You get the idea.
Remember that for the HPI (only 1 part of the history) it's possible to directly observe a complete HPI. For example: depressed skull fracture w/ loss of consciousness - severity, location, quality, associated signs.
Hope that helps.
F Tessa Bartels, CPC, CEMC