The term diagnostic test does not refer only to costly tests imaging or monitoring procedures like MRI, CT or cardiac catheterization. It refers in addition to the countless laboratory tests ordered everyday on patients such as electrolytes, serum chemistries, coagulation profiles or blood counts.
Understanding laboratory investigations provides nurses with essential and easily digested information about the most frequently requested tests that are performed on patients. It explains the physiological function being measured, details of reference ranges and clinical significance of test results for diagnosis or patient monitoring. Test results that warrant immediate clinical intervention are identified and wherever possible, abnormal test results are related to symptoms.
THE PROBABILISTIC NATURE OF DIAGNOSIS
Diagnosis is an uncertain art. This remains true in early phases of diagnostic evaluation. Studies of physicians reasoning have demonstrated that, as physician evaluate patients they keep a limited set of five to seven diagnostic hypotheses in mind, each of which is assigned, a relative probability such as 'very likely', 'possible', or 'unlikely'. These relative probabilities are adjusted upward or downward depending new information gained through history, physical examination and diagnostic tests. With enough information one diagnostic possibility becomes likely enough for 3the physicians to stop further investigations and declare that possibility as 'diagnosis'.
PURPOSES FOR ORDERING LABORATORY TESTS
The purposes for which tests are obtained have a great deal to do with both the choice of diagnostic tests and its interpretation. One survey of physicians in a large teaching hospital found that three general reasons accounted for most laboratory test ordering: diagnosis (37%), monitoring therapy (33%) and screening for asymptomatic disease (32%).
DIAGNOSIS
In order to use a test for diagnostic purposes, the test has to be positive in a large proportion of patients with disease- high sensitivity and negative in a large proportion with out the disease – high specificity. Ideally, for the test to be maximally useful for diagnostic use, both sensitivity and specificity should be 100%.
MONITORING THERAPY
Whenever a test is repeated in order to follow a therapeutic drug level or observe for side effects, it is being used for monitoring purposes.4
SCREENING
The object of the use of diagnostic tests for screening is to detect diseases in its earliest, presymp-tomatic state when, presumably, it is less widespread and more easily treated or cured. Most screening programs, such as occult blood screening or mammography are aimed at cancer detection.