FALTs: Factors Affecting Laboratory Tests

L. V. Rao

INTRODUCTION

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, doctoral-level 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. It is a complex and dynamic process differing from one hospital to another. It can cause random errors, undetectable by routine quality control and often unknown to testing personnel and clinicians. These factors may be divided further into those acting in vivo (biologic or physiologic) and those acting in vitro (specimen handling and interference factors). See Table 1-1 for some of the most frequently ordered tests, and their limitation causes false positive and false negative test results.

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 a 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.

TABLE 1-1. Preanalytic Factors Affecting Laboratory Tests

Test	Normal Range	Limitations	Falsely Increased	Falsely Decreased
ACTH	<46 pg/mL	Significant diurnal variations, high in the morning (6-8 AM). Releases in bursts, levels vary by minute.	Pregnancy, menstruation, stress, rheumatoid factor	Patients taking glucocorticoids
Albumin	0-4 mo: 2.0-4.5 g/dL 4 mo-16 y: 3.2-5.2 g/dL >16 y: 3.5-4.8 g/dL	Two dye-binding methods, bromocresol green (BCG) and bromocresol purple (BCP), are in routine use.	BCG overestimates albumin in serum due to poorer specificity for the analyte.	BCP underestimates in pediatric patients on hemodialysis and chronic renal failure.
Alkaline phosphatase	0-1 y: 150-350 IU/L 1-16 y: 30-300 IU/L >16 y: 30-115 IU/L	Day-to-day variations (5-10%) Higher in African American men (15%) and women (10%) compared to other racial/ethnic groups	Recent food ingestion, smoking, increased BMI (25%), drugs: penicillin, antiepileptic drugs, antihistamines, cardiovascular drugs	Use of oral contraceptives
AFP, tumor marker	0.6-6.60 ng/mL	Not recommended as a screening procedure to detect cancer in the general population	Tumors of the GI tract, liver damage (e.g., cirrhosis, hepatitis, or drug or alcohol abuse), and pregnancy Smoking tobacco (10%) Inflammatory bowel disease Some dietary supplements, herbal remedies, and vitamins can cause liver toxicity, leading to a rise in AFP levels.	NA
Ammonia	<50 µmol/L	Not useful to assess the degree of dysfunction (e.g., in Reye syndrome, hepatic function improves and the ammonia level falls, even in patients who finally die of these disorders) Levels are not always high in all patients with urea cycle disorders.	Atmospheric ammonia The presence of ammonium ions in anticoagulants High-protein diet, GI hemorrhage, increases due to cellular metabolism: 20% in 1 h and 100% by 2 h, prolonged tourniquet application	NA
ANA	Negative	Not recommended as a general screening test Depending on the titer, normal healthy population can have 5-20% positive test results.	Drugs (carbamazepine, chlorpromazine, ethosuximide, hydralazine, isoniazid, mephenytoin, methyldopa, penicillins, phenytoin, primidone, procainamide, and quinidine) Viral illness and infections	NA
Bilirubin total and direct	0-1 d: 0.0-6.0 mg/dL 1-2 d: 0.0-8.0 mg/dL 2-5 d: 0.0-12.0 mg/dL 5 d-4 mo: 0.3-1.2 mg/dL >4 mo: 0.3-1.2 mg/dL Direct bilirubin 0.0-0.4 mg/dL	Day-to-day variation (15-30%) Lower in African Americans	Fasting up to 48 h	Exposure to light Penicillin Acetyl salicylic acid Sulfisoxazole
Calcium, total	8.7-10.7 mg/dL	Need to interpret with total protein and albumin levels 0.8 mg of calcium is bound to 1.0-g albumin in serum. To correct, add 0.8 mg/dL for every 1.0 g/dL that serum albumin falls below 4.0 g/dL.	Hyperalbuminemia Multiple myeloma Waldenström macroglobulinemia Dehydration Venous stasis during blood collection by prolonged application of tourniquet Hyponatremia (<120 mmol/L)	Hypomagnesemia Hyperphosphatemia (e.g., laxatives, phosphate enemas, chemotherapy of leukemia or lymphoma, rhabdomyolysis) Hypoalbuminemia Hemodilution
CO₂, total	0-2 y: 20-25 mmol/L 2-16 y: 22-28 mmol/L >16 y: 24-32 mmol/L	80-90% present as bicarbonate and is general guide to body’s buffering capacity	Antacids Corticotrophin Mercurial and thiazide diuretics Sodium bicarbonate Hyperthermia	Acetazolamide Ammonium chloride, aspirin, chlorothiazide diuretics, methicillin Paraldehyde, tetracycline High altitudes
CA-125	0-35 U/mL	Not recommended for screening asymptomatic women Not increased in mucinous adenocarcinoma Different manufacturer assays do not produce similar value. Should not be used interchangeably.	Pregnancy Menstruation, endometriosis Pleural effusion or inflammation Peritoneal effusion or inflammation, cirrhosis, severe liver necrosis (66%), disorders of the GI tract, liver, and pancreas Renal failure, healthy persons (1%)	Postmenopausal women African American and Asian women
Cholesterol, total	<200 mg/dL	Intraindividual variation (10%) Seasonal variation (8%) higher in the winter than in summer Positional variation is 5% and 10-15% lower when phlebotomized sitting or recumbent, respectively, as opposed to standing.	Pregnancy, drugs: beta blockers, anabolic steroids, vitamin D, oral contraceptives, and epinephrine Smoking, alcohol, renal failure Hypothyroidism, glycogen storage disease, biliary cirrhosis, hepatocellular disease Prostate and pancreatic neoplasms	Acute illness such as a heart attack Malnutrition Liver disease Myeloproliferative diseases Chronic anemias Infection Hyperthyroidism Stress
CRP	<10 mg/L	Not a screening test for wellness and should only be used in the diagnosis and monitoring of a patient who appears to have an inflammatory process Elevated levels are nonspecific and should not be interpreted without a complete clinical history. Influenced by genetics, age, and sedentary lifestyle, stress, exposure to environmental toxins, and diet that specifically contains refined, processed, and manufactured foods	Oral contraceptives Pregnancy Monoclonal gammopathy	Patients treated with carboxypenicillins Liver failure
ESR	0-15 mm/h in men 0-20 mm/h in women	Not a good screening test Compared to the ESR, CRP is a more sensitive and specific marker of the acute phase reaction and is more responsive to changes in the patient’s condition. There are only two circumstances where the erythrocyte sedimentation rate is superior—detecting low-grade bone and joint infections and monitoring disease activity in systemic lupus erythematosus.	Increased fibrinogen; increased gamma- and beta globulins Drugs (dextran, penicillamine, theophylline, vitamin A, methyldopa, methysergide) Technical factors (e.g., hemolyzed sample, high temperature in the laboratory) Hypercholesterolemia	Abnormally shaped RBCs (sickle cells, spherocytes, acanthocytes) Microcytosis, HbC disease, hypofibrinogenemia Technical factors (low temperature in the laboratory, clotted blood) Extreme leukocytosis Drugs (quinine, salicylates, high steroid levels, drugs that cause high glucose levels)
Ferritin	Male: 23-336 ng/mL (in patients with normal iron stores, it should be >30 ng/mL) Female: 11-306 ng/mL	In hepatic, malignant, and inflammatory conditions, ferritin levels can be normal. In such cases, bone marrow stain of iron may be used to exclude iron deficiency.	Inflammation Liver and kidney disease, malignancy Obesity and age Alcohol use	Vegetarianism
GGT		Sensitive but nonspecific indicator of biliary disease Half-life (7-10 d); in alcoholassociated liver injury, the half-life is increased to as much as 28 d, suggesting impaired clearance. Day-to-day variations (10-15%); approximately double in African Americans	Higher BMI (25-50%) Renal disease Cardiac disease Phenytoin	Pregnancy (25%)
HCG	Negative	Approximately one third of all conceptions end in natural termination. This may produce positive results when testing early in the pregnancy followed by negative results after the natural termination.	Marijuana use Human antimouse antibody (HAMA) or heterophilic antibody Elevated LH levels due to low testosterone can lead to false positives. Duodenal ulcers Cirrhosis of the liver Inflammatory bowel disease	Smoking tobacco (20%) False negatives in urine tests can be caused by drinking large amounts of fluids and taking diuretics, anticonsultants, anti-Parkinson drugs, tranquilizers, hypnotics, or certain antihistamines before the test.
Lipase	0-50 U/L	More specific for pancreatitis than is for serum amylase; diagnosis of peritonitis, strangulated or infarcted bowel, pancreatic cyst	Cholinergic drugs, opiates Alcohol Renal disease Macrolipasemia	Citrates Intravenous dextrose Oxalates Saquinavir
PTH-intact	12-65 pg/mL	Inherent biologic variability due to hormone pulsatility and diurnal variation, choice of tube for sample collection, as well as sample stability at storage temperature Because of a pronounced nocturnal rise in intact PTH levels observed in a small experimental male population, sampling after 10 AM for optimum discrimination between normal and those with mild primary hyperparathyroidism has been suggested.	Heterophile antibodies Rheumatoid factor	Sedative-hypnotic drug propofol (Diprivan)
PSA	<4 ng/mL	Ambulatory values are higher than sedentary values, which may decrease ≤50% (mean = 18%). Methodologic variations exist depending upon calibrators (WHO vs. Hybritech) used by various manufacturers (22% average bias).	Prostatic massage, ≤2 times Cystoscopy: 4 times Needle biopsy: >50 times for ≤1 mo Transurethral resection: >50 times, digital rectal examination, indwelling catheter Vigorous bicycle exercise: ≤2-3 times several days, ejaculation within 24-48 h Urinary tract infection	Castration Antiandrogen drugs (e.g., finasteride) Radiation therapy Prostatectomy PSA falls 17% in 3 d after lying in hospital
TSH	0.5-6.3 µIU/mL, depending on age and sex	Not be useful for hospitalized ill patients. Has a diurnal rhythm (50%), with peaks at 2:00-4:00 AM and troughs at 5:00-6:00 PM with ultradian variations	Dopamine Glucocorticoids Rheumatoid factor Human antimouse antibodies Heterophile antibodies Thyroid hormone autoantibodies Macro-TSH	Severe dehydration Biotin may cause falsely low values in immunometric assays.