How does a lack of information impact clinicians in assigning specific diagnoses of their patients' conditions?

No nurse or physician can be expected to make a completely accurate diagnosis of a patient's condition absent the relevant information. While it seems tautological to suggest that such a situation would never arise, that is far from true. Hospital emergency rooms frequently receive patients delivered by ambulance or by police the identification of whom is unknown for an hour or more. As such, there is no case history to accompany this "John Doe" patient from which attending medical staff can begin to assess the unidentified patient's condition. In the case of a shooting or stabbing victim, the ailment is obvious. In cases in which an unidentified patient is suffering from an unknown illness, however, it is incumbent upon medical staff to make a preliminary assessment based solely upon the patient's "vitals," such as blood pressure and body temperature. Bodily fluids and other substances, especially blood, but also urine and feces, can provide vital information, but only after these substances are drawn and analyzed by phlebotomists and laboratory personnel.

When patients enter hospitals, whether through emergency care or on the recommendation of attending or primary care physicians, their personal medical histories are generally easily accessible. Those histories, which include known allergies to medications and food as well as prior instances of illnesses and/or surgical procedures, provide medical personnel the background they need to assess the current condition of the patient. The more background information medical staff has, and the more contemporaneous information attained through examination of the patient, the more accurate the diagnosis. 

The human body is, needless to say, an extraordinarily complex "system of systems," in which a flaw in or injury to any one part can affect the well-being of the individual to greater or lesser degrees. As noted, blood samples can provide a great deal of vital information, as can imaging of the pertinent part of the patient's anatomy. Information obtained through the use of magnetic resonance imaging, or MRI, for example, or through CT scans is often essential for the diagnosis of many ailments, including tumors or muscle or other tissue tears. That is why advances in diagnostic technologies have proven so important  to so many people. A diagnosis based upon incomplete information can prove catastrophic. There is often little margin for error in the diagnosis of a patient's condition, and the more information available to medical staff, the more likely the diagnosis will be accurate, and the proper course of treatment can be prescribed.