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C HAPTER 11 / Laboratory Tests Using Blood 247
versy still exists as to whether aspirating blood from an IV patient has assumed the sitting position for at least 15 minutes
catheter or saline lock provides less hemolysis than venipuncture (from a standing position), and in some cases 20 to 30 minutes.
4
with a needle. Dugan et al. found drawing blood from a size In some settings, such as the hospital, it is not difficult to stabilize
22-gauge IV catheter caused the most hemolysis in their emer- the patient’s position and thus reduce variability. In other settings,
6
5
gency department. Lowe et al. and Grant found that venipunc- such as ambulatory care, significant variability is introduced if the
7
ture had less hemolysis than IV catheters, Kennedy et al. found patient is not made to sit for at least 15 minutes before the blood
larger size IV catheters caused less hemolysis than smaller sized draw. Because control over sitting time is not usually feasible or
8
catheters, and Cox et al. found using a 5-ml vacuum collection practical, care should be taken in the interpretation of results. Ex-
tube demonstrated better results than 10-ml vacuum tubes. ercising immediately before blood sample collection frequently
9
However, Corbo et al. and Sliwa 10 determined aspirating the produces significantly erroneous results, especially with enzyme
sample from a saline lock after discarding blood did not result in evaluation. Forearm exercises before blood withdrawal may lead
more hemolysis than venipuncture. Arrants et al. 11 found similar to hemolysis.
results using 18-gauge saline locks for use with coagulation stud- The timing of blood sampling should include consideration of
ies. These studies demonstrate how important technique is to the effect of medications on the interpretation of results. Med-
specimen collection. ications affect results of many specimens drawn for chemistry,
Proper specimen collection includes accurate identification of hematology, coagulation, hormonal, and enzyme studies. Knowl-
the patient and accurate labeling of the specimen at the site of col- edge of the effect of the drug assists in proper timing or subse-
lection. It also includes rapid transport to the laboratory, because quent interpretation of the results. Consideration should also be
cells remain viable after collection and continue their metabolic given to the effects of other influences, including over-the-counter
processes. Specimens that are left to stand unprocessed often yield medications, caffeine, nicotine, ethanol, home remedies, and
inaccurate results. herbal therapies.
In therapeutic drug monitoring, blood drug levels are moni-
Interpretation of Results tored to evaluate the effects of drug therapy, make decisions re-
garding dosage, prevent toxicity, and monitor patient adherence.
Inherent physiologic variability exists based on patient age, sex, Timing of the blood sample usually depends on the half-life of the
ethnicity, and health status (such as, pregnancy or post-myocar- drug; samples drawn at projected peak level assist in monitoring
dial infarction [MI]). These physiologic differences affect inter- for toxicity, whereas levels drawn at trough help to verify the min-
pretation of results. Physiologic changes associated with the ag- imum satisfactory therapeutic level for that patient. Regardless of
ing process bring concomitant changes in some expected the purpose of the blood sample, drugs that may affect interpre-
laboratory results. Because men usually have more muscle mass tation of results should be noted on the laboratory slip. For ther-
than women, gender differences are seen in substances related to apeutic drug monitoring, it is important to note the date and time
muscle function or metabolism, such as creatinine. There may of the last dose as well.
be significant differences among European, African, and Asian Different laboratories use different equipment and methods
populations in testing for cholesterol, enzymes, and hormones. by which to test specimens. Specific reference ranges are usually
Various physiologic states, such as pregnancy, stress, obesity, and reported alongside the patient’s results on the laboratory
endurance exercise, also introduce situational changes in ex- report. In an effort to establish a standard for communicating
pected results. 12 laboratory results, the World Health Organization has recom-
Cyclic variability produces daily, monthly, or yearly patterns mended that the medical and scientific community throughout
in physiologic states. These cycles are often taken into consider- the world adopt the use of the International System of Units
ation in the collection or interpretation of laboratory results. 12 (ISU). An international unit is defined as the number of moles
As a result, most routine specimens, at least in the hospital set- of substrate converted per second under defined conditions.
ting, are drawn in the early morning to control for any circadian Thus, many laboratories may report results in different
variability. ways, depending on their accepted standard of practice. Most
Blood tests are sometimes affected by the ingestion of food or laboratories also report critical (or panic) values. These values
fluids. Not only are results affected by the absorption of dietary should be reported promptly to the provider so that results
components into the blood after a meal, but hormonal and meta- may be evaluated (and decisions made) in light of the patient
bolic changes occur as well. Partial control for the variability in- condition.
troduced by food or fluid ingestion can be achieved either by Most reference ranges have been established for venous
drawing early morning, pre-meal specimens, or by having the pa- blood samples. Because arterial blood has higher concentrations
tient fast for 8 to 12 hours. The latter is especially important in of glucose and oxygen and lower concentrations of waste prod-
lipid testing. 13 ucts (i.e., ammonia, potassium, and lactate), an arterial source
Sometimes, differences based on position are negligible. In (instead of venous) should be noted on the laboratory slip. Cap-
other cases, they are significant. Patient position during (and be- illary samples yield results that are closer to arterial blood than
fore) sampling can affect results. In the upright position, there venous.
may be a shift in extracellular fluid volume into the tissues. With Critical evaluation of laboratory results should take into
the resulting increased concentration of proteins and protein- account how the reference or “normal” values were determined.
bound substances in the vascular space, samples for proteins, Patients who have been seen for a long time by the same provider,
enzymes, hematocrit (Hct), hemoglobin (Hb), calcium, iron, or those who have been seen within the same health care organi-
hormones, and several drugs may show an average 5% to 8% in- zation, sometimes establish their own reference range. Reference
crease. Redistribution of extracellular fluid volume and ranges for a specific disease are sometimes established through
electrolytes within the vascular space does not stabilize until a large-scale clinical trials.