Use of Tests In Diagnosis and Management

The value of a test in a particular clinical situation depends not only on the test’s sensitivity and specificity but also on the probability that the patient has the disease before the test result is known (pretest probability).The results of a valuable test will substantially change the probability that the patient has the disease (posttest probability). The pretest probability of disease has a profound effect on the posttest probability of disease.

Test Results

It is important to understand normal or reference values.

-Normal ranges can vary to some degree from laboratory to laboratory. Frequently, this is because of the particular type of equipment used. Theoretically, normal can refer to the ideal health state, to average reference values, or to types of statistical distribution.

-The reported reference range for a test can vary according to the laboratory used, the method employed, the population tested, and methods of specimen collection and preservation.

-Most normal blood test values are determined by measuring fasting specimens.

-Specific factors can influence test results. For example, patient posture is important when plasma volume is measured because this value is 12% to 15% greater in a person who has been supine for several hours. Changing from a supine to a standing position can alter values as follows: increased hemoglobin (Hb), red blood cell (RBC) count, hematocrit (Hct), calcium (Ca), potassium (K), phosphorus (P), aspartate aminotransferase (AST), phosphatases, total protein, albumin, cholesterol, and triglycerides. Going from an upright to a supine position results in increased hematocrit, calcium, total protein, and cholesterol. A tourniquet applied for more than 1 minute produces laboratory value increases in protein (5%), iron (6.7%), AST (9.3%), and cholesterol (5%) and decreases in K+ (6%) and creatinine (2% to 3%).

Laboratories must specify their own normal ranges. Many factors affect laboratory test values and influence ranges. Thus, values may be normal under one set of prevailing conditions but may exhibit different limits in other circumstances. Age, gender, race, environment, posture, diurnal and other cyclic variations, foods, beverages, fasting or postprandial state, drugs, and exercise can affect derived values. Interpretation of laboratory results must always be in the context of the patient's state of being. Circumstances such as hydration, nutrition, fasting state, mental status, or compliance with test protocols are only a few of the situations that can influence test outcomes.

Laboratory Reports

Scientific publications and many professional organizations are changing clinical laboratory data values from conventional units to International (SI) units. Currently, many data are reported in both ways.

The SI system uses seven dimensionally independent units of measurement to provide logical and consistent measurements. For example, SI concentrations are written as amount per volume (moles or millimoles per liter) rather than as mass per volume (grams, milligrams, or milliequivalents per deciliter, 100 milliliters, or liter). Numerical values may differ between systems or may be the same. For example, chloride is the same in both systems: 95 to 105 mEq/L (conventional) and 95 to 105 mmol/L (SI).