We ask the patient for their "subjective" report that we, as medical experts, need to validate through the diagnostic process.
But, "objective" tests may be biased also.
X-rays are commonly referenced to illustrate the difficult dichotomy between "soft", subjective data and "hard", objective data.
When we make decisions we would like to have the high quality data, and then, if possible, consider other, lower quality data.
In diagnosing an x-ray, radiologists decision making is made up of two parts: Perception and Cogniton.
Perception is the physicians' eyesight, image quality, stage of pathology, training, experience, and possibly the prevalence of the condition in the population. These are all well-measured factors that we train in medical school, certify by government agencies or enforce through legal practice standards.
Cognition is less well-measured. Cognition varies by individual and, within individuals, cognition varies by time of day, mood, fatigue, knowledge of other patient factors, stress, time frame of the decision, setting of the physician and other "soft" factors.
Radiologists and X-ray Findings
Radiologists have a difficult job. In one study of mammography, their inter-observer reliability was only 48%. That's "moderate".
See this Medscape slideshow of difficult-to-interpret X-ray images of the lung.
Slide 13 of Easy-to-Miss Findings on Chest Radiographs shows a wedge thoracic compression fracture that can be missed on x-ray imaging.
A newly published decision rule by Henschke enables physical therapists to screen for vertebral fractures without the need for expensive and irradiating images.
Slide 14 of Can't Miss Critical Findings on Plain Chest Radiography shows a pulmonary emboli that broke off from a deep vein thrombosis in the leg.
"Of note, most chest radiographs are normal in patients with a pulmonary embolism".Physical therapists can detects thrombotic blood formations before they turn into a hard-to-catch, deadly emboli using Well's criteria that will find blood clots with greater sensitivity that a physician's clinical intuition.
Physical Therapists' Findings
Even commonly used physical therapists' tests traditionally described as "objective" are less than helpful. Commonly used tests taught to orthopedic manual therapists for sacro-iliac dysfunction and SLAP tears have little predictive value.
In some cases, a negative test may be able to rule out the condition but cannot confirm the diagnosis. Physical Therapists Chad Cook and Eric Hegedus have written this excellent book to clarify some of the issues surrounding diagnostic accuracy.
However, some "subjective" tests are reliable most of the time. Consider that the test-retest reliability of the Oswestry Disablement Index is 0.99. Almost perfect.
In the proper selection of "objective" physical therapy tests and measures patient bias seems to be less likely than clinician bias.