Reference Range

Laboratory tests are employed to determine a patient’s health status and aid medical decisions. It is important to determine whether a specific test result falls within an expected range of values considered to be “normal.” The notion of normal is problematic, however. Early on in the history of laboratory medicine, tests to determine the normal range for blood chemistry and hematology were done primarily on small convenience samples of subjects who were not representative of the larger population in terms of age, gender, race, and ethnicity. An additional problem is that the term normal is not the same as healthy. The best example is cholesterol. A normal range of cholesterol found in most Americans puts them at risk of cardiovascular disease and cannot be considered healthy.

Beginning in the 1970s,¹ the term normal ranges was slowly replaced with more appropriate terms such as reference ranges, biologic reference intervals, and expected value.² This change in terminology acknowledged that what we consider normal must take into account variations related to age, gender, race, and ethnicity, which change over time as the demographic composition of society changes. A reference range sets the boundaries for any analyte (e.g., electrolyte, blood cell, protein, enzyme) that would likely be encountered in healthy subjects. This range would encompass the variability reflected in the larger, presumably healthy population.

Reference ranges vary from laboratory to laboratory for various reasons, including differences in measurement techniques, the populations of healthy individuals used to establish the reference intervals, or analytic imprecision when the intervals were constructed. Most differences in reference ranges are relatively small, with reasonably close agreement between most laboratories.² The reference ranges and critical values given in this chapter are from a single institution, and they serve as representative examples. RTs must become familiar with the reference ranges used at their institutions.

Critical Test Value

A critical test value is a result significantly outside the reference range and represents a pathophysiologic condition. A critical value may be potentially life-threatening unless corrective action is taken promptly. Critical values are reported in the hospital to alert caregivers as well to decrease medical errors and protect patients.

Typically, critical values are communicated by telephone from the clinical laboratory to the general ward or intensive care unit where the patient is situated. The nurse or RT who receives these results is required to read-back the critical value to the clinical laboratory. This requirement is to ensure that the correct information has been communicated. It is the responsibility of the nurse or RT to communicate the critical value in a timely fashion to the physician caring for the patient. The same read-back procedure is used. All communication of critical test values is documented in the medical record.

In this chapter, critical values are listed along with common pathophysiologic states with which they commonly occur. Not all clinical analytes have an associated critical value. For some tests, there is no general agreement on what a critical value would be. Others have only a one-sided value that exists below or above a critical threshold; this is true particularly for substances that do not normally appear in the blood. For example, certain enzymes and proteins are released only after extensive cellular damage following injury (see later section on enzyme tests). Under normal circumstances, these proteins or enzymes may be virtually undetectable in the serum or plasma.

When interpreting derangements for any test result, the clinician must consider the context of the change. In a patient with chronic renal disease, a serum creatinine of 3.0 mg/dl (approximately twice the upper limit of normal) would not be considered urgent. However, in a patient who presents with a bloodstream infection (i.e., sepsis) and hypotension, a sudden increase in serum creatinine to 3.0 mg/dl would be considered critical because it indicates acute kidney injury in the context of rapidly developing clinical instability.

White Blood Cell Count

Red Blood Cell Count

The primary function of RBCs or erythrocytes is to supply oxygen to the tissues. The RBC count helps determine the ability of the blood to carry oxygen. An abnormally low RBC count is referred to as anemia and suggests that either RBC production by the bone marrow is inadequate or excessive blood loss has occurred. In either case, the oxygen-carrying capacity of the blood is reduced, and the patient is more likely to experience tissue hypoxia. There are several types of anemia with different causes. Some are related to dietary deficiencies in iron or vitamins (e.g., vitamin B₁₂ and folate). Other causes are related to chronic inflammatory diseases, such as Crohn disease, HIV/AIDS, lymphoma, and autoimmune diseases that result in the destruction of erythrocytes (hemolytic and aplastic anemia). A hereditary cause is sickle cell anemia, which is common in African-Americans. For most forms of anemia, a blood transfusion may be needed if the RBC count is too low. The trigger point for transfusion is based on the hemoglobin or hematocrit measurement rather than the RBC count.

An abnormally elevated RBC count is known as polycythemia. It occurs most often when the bone marrow is stimulated to produce extra RBCs in response to chronically low blood oxygen levels (secondary polycythemia). Polycythemia counteracts the negative side effects of reduced PO₂ in the blood by increasing the oxygen-carrying capacity of the blood. Patients who live at a high altitude and patients with chronic lung disease are most likely to experience chronic hypoxia and to develop secondary polycythemia.

In addition to the RBC count, the clinical laboratory reports hemoglobin and hematocrit levels. Hemoglobin is a protein substance with the unique ability to bind with oxygen. Each healthy RBC contains 200 million to 300 million molecules of hemoglobin, for a hemoglobin level of 12 to 17 g/dl in a healthy adult. Patients with an inadequate hemoglobin concentration have reduced oxygen-carrying capacity. In this condition, the RBCs are smaller than normal (microcytic anemia) and lack normal color (hypochromic anemia). The necessity for RBC transfusion depends on the cause of anemia and the patient’s overall condition. Usually a transfusion is triggered by a hemoglobin concentration of approximately 7.0 g/dl or a hematocrit of approximately 21%.