Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (3 page)

CHAPTER
15  Transfusion Medicine
Vishesh Chhibber

SECTION 2 LAB TESTS

CHAPTER
16  Laboratory Tests
Lokinendi V. Rao and Liberto Pechet

CHAPTER
17  Infectious Disease Assays
Michael J. Mitchell and Lokinendi V. Rao

APPENDIX     
Abbreviations and Acronyms

Index

Introduction

Chapter 1

FALTs: Factors Affecting Laboratory Tests

Lokinendi V. Rao

What Causes Abnormal Test Results (Besides Disease)?

Preanalytic Errors

Physiologic Factors

Specimen Handling Factors

Analytic Errors

Diagnostic Test Values

Accuracy and Precision

Receiver Operating Characteristic (ROC) Curves

Postanalytic Errors

Reference Intervals

Performing the Right Test at the Right Time for the Right Reason

Laboratory testing is an integral part of modern medical practice. Although clinical laboratory testing accounts for only 2.3% of annual health care costs in the United States, it plays a major role in the clinical decisions made by physicians, nurses, and other health care providers for the overall management of disease. More than 4,000 laboratory tests are available for clinical use, and about 500 of them are performed regularly. The number of Clinical Laboratory Improvement Amendments (CLIA)-certified laboratories has grown to exceed more than 200,000. The laboratory medicine workforce comprises pathologists, doctorallevel laboratory scientists, technologists, and technicians, who play a vital role in the health care system.

The health care system is increasingly dependent on reliable clinical laboratory services; however, as part of the overall health care system, these laboratory evaluations are prone to errors. Laboratory medicine comprises more than just the use of chemicals and reagents for the measurement of various analytes for clinical diagnosis purposes. Interference by both endogenous and exogenous substances is a common problem for the test analysis. These substances play a significant role in the proper interpretation of results, and such interference is adverse to patient care and adds to the cost of health care. It would be an oversimplification to conclude that each variable will always produce a specific effect; it depends on the person, the duration of exposure to that variable, and the time between initial stress, the sample collection, and the degree of exposure. Awareness that many factors occurring outside the laboratory in and around the patient may affect the test result before the sample reaches the laboratory or even before the sample is collected is very important. These factors can be minimized when the clinician takes a good history and when there is a good communication of such information between the laboratory and the physician.

   
WHAT CAUSES ABNORMAL TEST RESULTS (BESIDES DISEASE)?

The total testing process defines the preanalytic, analytic, and postanalytic phases of laboratory testing and serves as the basis for designing and implementing interventions, restrictions, or limits that can reduce or remove the likelihood of errors. Over the last several years, there has been a remarkable decrease in error rates, especially analytic errors. Evidence from recent studies demonstrates that a large percentage of laboratory errors occur in preanalytic and postanalytic steps. Errors in the preanalytic (61.9%) and postanalytic (23.1%) processes occurred much more frequently than occurrences of analytic errors (15%). About one fourth of these can have consequences to the patient either in delay of the test result or life threatening.

PREANALYTIC ERRORS

Preanalytic factors act on both the patient and the specimen before analyses. These factors may be divided further into those acting in vivo (biologic or physiologic) and those acting in vitro (specimen handling and interference factors).

PHYSIOLOGIC FACTORS

Some physiologic factors are beyond our control. They include age, sex, and race, and so on, and can be managed by placing appropriate reference limits. Others factors such as diet, starvation, exercise, posture, diurnal and seasonal variations, menstrual cycle, and pregnancy must be considered in the interpretation of the test results. Age has noticeable effect on some test results and the need for establishing appropriate reference intervals. In newborns, the composition of blood is affected by the maturity of the infant at birth. At birth, RBC and hemoglobin values are higher than adults due to low levels of oxygen in the uterus. They continue to decrease and level out to adult values about the age of 15. Adult values are usually taken as the reference for comparison with those of the young and the elderly. The concentration of most test constituents remains constant between puberty and menopause in women and between puberty and middle age in men. The plasma concentrations of many constituents increase in women after menopause. Hormone levels are affected by aging. However, changes in concentrations are much less pronounced than an endocrine organ’s response to stimuli. Until puberty, there are few differences in laboratory data between boys and girls. After puberty, the characteristic changes in the levels of sex hormones become apparent.

In addition to the commonly known hormonal changes during the menstrual cycle, there is a preovulatory increase in the concentrations of aldosterone and renin. Coincident with the ovulation, serum cholesterol levels are lower than at any other phase of the menstrual cycle. In pregnancy, a dilutional effect is observed due to the increase in mean plasma volume, which in turn causes hemodilution. Normal pregnancy is characterized by major physiologic adaptations that alter maternal blood chemistry and hematology laboratory values. In addition, there are time-related fluctuations in the levels of certain analytes. Many analytes—such as cortisol, thyrotropin (TSH), growth hormone, potassium, glucose, iron, and proinflammatory cytokines—exhibit diurnal variation. Hormones such as luteinizing hormone (LH), follicle-stimulating hormone (FSH), and testosterone released in short bursts lasting barely 2 minutes make accurate measurements problematic. Seasonal changes also affect certain analytes like vitamin D (higher during summer), cholesterol, and thyroid hormones (higher during winter). Changes in the levels of some of the constituents in blood occur when measured at sea level as opposed to measurement at a higher altitude. Hematocrit, hemoglobin, and C-reactive protein (CRP) can be higher at high altitude. Levels of plasma renin, transferrin, urinary creatinine, creatinine clearance, and estriol decrease with increasing altitude.

The dietary effect of laboratory test results is complex and simply cannot be separated into the categories of “fasting” and “nonfasting” status of the patient. A significant variation in some routine tests after a regular meal showing that fasting time needs to be carefully considered when performing tests to prevent spurious results. Clinically significant differences are observed within 1–4 hours for triglycerides, albumin, ALT, calcium, iron, LDH, phosphorus, magnesium, lymphocytes, RBC, hemoglobin, hematocrit, etc. The type of diet (high fat, low fat, vegetarian, malnutrition), length of time since last meal, and test-specific dietary concerns can affect some tests. Consumption of caffeine, bran, serotonin (consumption of fruits and vegetables such as bananas, avocados, and onions), herbal preparations (e.g., aloe vera, Chinese rhubarb, senna, quinine, and quinidine), recreational drug use, ethanol, and smoking can induce both short- and long-term effects that alter the results of several analytes. Differentiations of the effects of race from those of socioeconomic conditions are difficult. Carbohydrate and lipid metabolism differ in blacks and whites. Glucose tolerance is less in blacks, Polynesians, and Native Americans in comparison to whites.

Physical stress and mental stress influence the concentrations of many plasma constituents, including cortisol, aldosterone, prolactin, TSH, aldosterone, cholesterol, glucose, insulin, and lactate. With blindness, the normal stimulation of the hypothalamic–pituitary axis is reduced. Consequently, certain features of hypopituitarism and hypoadrenalism may be observed. In some blind individuals, the normal diurnal variations of cortisol may persist; in others, it does not. Fever provokes many hormonal responses as does shock and trauma. The stress of surgery has been shown to reduce the serum triiodothyronine (T
₃
) levels by 50% in patients without thyroid disease.

Transfusions and infusions can also significantly affect the concentration of certain laboratory values. For persons receiving an infusion, blood should not be obtained proximal to the infusion site. Blood should be obtained from the opposite arm. A minimum of 8 hours must elapse before blood is obtained from a subject who has received fat emulsion. For patients receiving blood transfusions, the extent of hemolysis and with it increased levels of potassium, lactate dehydrogenase (LDH), and free hemoglobin are released progressively to the age of the transfused blood.

Exercise such as running up and down several flights of stairs or strenuous activity such as working out in a gymnasium or marathon running the night before the specimen collection can affect the results obtained for several analytes. To minimize preanalytic variables introduced by exercise, subjects should be instructed to refrain from strenuous activity on the night before testing and not to exert themselves by walking a long distance, running, or climbing stairs before blood specimen collection. In addition, the muscle damage associated with trauma of surgery will increase the serum activity of enzymes originating in skeletal muscles, and this activity may persist for several days.

The plasma and extracellular volumes decrease within a few days of the start of bed rest. With prolonged bed rest, fluid retention occurs and plasma protein and albumin levels may be decreased by an average of 0.5 and 0.3 g/dL, respectively. As a result, concentrations of bound protein are also reduced. Changes in posture during blood sampling can affect the concentrations of several analytes measured in serum or plasma. Change in posture from a supine to an erect or sitting position can result in a shift in body water from intravascular to interstitial compartments. As a result, the concentrations of larger molecules that are not filterable are increased. These effects are accentuated in patients with a tendency for edema, such as in cardiovascular insufficiency and cirrhosis of the liver.

SPECIMEN HANDLING FACTORS

Among controllable preanalytic variables, specimen collection is most critical. Unacceptable specimen collection due to misidentification, insufficient volume to perform test, incorrect whole blood-to-anticoagulant ratio, and specimen quality (hemolysis, clots, contaminated, collected in wrong container) accounts for the majority of preanalytic errors. Hemolysis, lipemia, and icteric samples have variable effects on assays and depend upon testing method and analyte. The time and temperature for storage of the specimen and the processing steps in the preparation of serum or plasma or cell separation can introduce preanalytic variables.

Pneumatic tube systems of various lengths are routinely used in many hospitals to transport blood collection tubes to the testing laboratory. Significant differences between plasma but not serum levels of LDH are observed when blood collection tubes are transported through pneumatic tube systems. The application of a tourniquet, by reducing the pressure below the systolic pressure, maintains the effective filtration pressure within the capillaries. As a result, small molecules and fluid are transferred from the intravasal space to the interstitium. Application of tourniquet for longer than 1 minute can result in hemoconcentration of large molecules that are unable to penetrate the capillary wall. To minimize the preanalytic effects of tourniquet application time, the tourniquet should be released as soon as the needle enters the vein. Avoidance of excessive fist clenching during phlebotomy and maintaining tourniquet application time to no more than 1 minute can minimize preanalytic errors.