Blood Sample Collection
General guidelines for blood sample collection have been developed that help to ensure patient and clinician safety and maximize interpretation of results. The clinician should consider the policies and procedures of his or her organization with regard to blood specimen collection, as well as the standards for professional, national, and international organizations. Control of variability can be enhanced by use of proper technique during collection and processing of the specimen.
During specimen collection through venipuncture, the use of a tourniquet produces changes within the vein. Once a tourniquet is placed on the arm, veins dilate because of their inability to drain. Cellular injury and hemolysis can be caused by the prolonged use of a tourniquet, described as 3 minutes or longer. Under these conditions, return of fluid and electrolytes to the vein is decreased or prohibited, resulting in a hemoconcentrated specimen. In addition, despite decreased circulation of fresh blood to the tissues, cells continue their metabolic processes, leading to an increased concentration in metabolic waste products, such as lactate. In this more acidic environment, potassium leaks out of cells. In general, the tourniquet should not be left on more than 1 minute.2
Longer use may be unavoidable during a difficult venipuncture. In such cases, information about a difficult venipuncture should be noted on the laboratory slip to assist with interpretation of results.
Blood specimens can be contaminated in several ways. During collection, contamination may occur from intravenous (IV) fluids. Blood draws should not be done on the same arm as an infusion. If the infusion arm cannot be avoided, a tourniquet may be placed between the IV site and the phlebotomy site. Slowing the IV to a keep-open rate (if not contraindicated) for 3 to 5 minutes before the draw may help to reduce contamination of the blood sample. In any event, it should be noted on the laboratory slip that the sample was obtained under these conditions.
Contamination may also be introduced by improper use of blood tubes. Most specimen collection tubes contain some form of anticoagulant. If blood has been mistakenly collected in one tube containing anticoagulant, it should never be poured into a different tube. Also, blood entering one tube should never be allowed to contaminate remaining blood that will be introduced into another tube.
Another source of contamination is introduced when routine samples are drawn from arterial lines, vascular catheters, or
ports. The use of an indwelling intravascular catheter allows access to the patient’s blood supply without further invasive procedure. Comfort for the patient, ease, and speed of periodic specimen collection are some of the benefits of using an intravascular line for blood sampling. Intravascular catheters may be kept patent by continuous or intermittent infusion, or by instilling saline or heparin solutions. Sometimes, solutions delivered through the catheter may contain medications. The infusate or any additives may dilute blood constituents. This dilution would have the effect of lowering the concentration of the desired sample.
The diameter and length of the catheter are important determinants when collecting blood specimens. The institution’s policy and procedure manual should be consulted for recommended withdrawal and discard from vascular devices. Additional sources, such as professional nursing society standards, may assist in making decisions about recommended discard volumes.
Whether sampling from a pulmonary artery catheter, central venous line, arterial catheter, or other intravascular catheter, attention should be paid to the feasibility of interruption of the system. The pulmonary artery catheter presents particular problems. Both the right atrium (RA) port and venous infusion port (VIP) are useful for the administration of drugs and fluids. Although the RA port allows access to the central circulation, use of this port for thermodilution cardiac output calculations makes it difficult to infuse drugs or fluids (a large amount of the infusate might be delivered during delivery of the cardiac output injectate). Consequently, the VIP is chosen for fluid and drug infusion. In this situation, the use of the RA port may be preferable for blood withdrawal. If vasoactive drugs are not infusing through the VIP, it may also be used for blood sampling. The proximal opening into the RA is upstream of any drugs or fluid infusing through the RA port; therefore, the possibility of contamination of the blood sample by infusates is minimized. Typically, the distal port of the pulmonary artery catheter is only used for blood sampling when measuring a mixed venous blood gas from the pulmonary artery where venous blood mixes after circulating through the superior and inferior vena cavae, coronary sinuses, and the chambers in the right side of the heart.
Many institutions have shifted to the use of saline in lieu of heparin in IV lines and pressure tubing however, questions persist about appropriate discard from vascular catheters (instilled with heparin) when drawing blood for coagulation studies. Inconsistent results may increase the cost to the patient through repeated testing, wasted blood, or erroneous treatment decisions. Again, the nurse may refer to policy and procedure manuals or professional organization standards for guidance. In any event, coordination in obtaining multiple blood specimens decreases the amount of blood that is eventually discarded and the number of times that the sterile system is invaded, thus reducing risk of introducing infection.
Sepsis has been associated with intravascular monitoring equipment including stopcocks and pressure transducers, which may be the most frequent reservoirs for endemic contamination. The incidence of local infection and bacteremia has been reduced by the use of disposable transducers and the percutaneous sheath systems used to introduce pulmonary artery catheters. The withdrawal of blood from an intravascular catheter should be considered a sterile procedure. Once removed, caps used to cover stopcock openings should always be replaced with a sterile cap. The person performing the procedure should be gloved. Syringes, used once, should be discarded.
Types of Specimens
When blood is withdrawn from the body, it eventually clots. The fluid that separates from the clot is called serum.
Plasma, from unclotted blood, contains fibrinogen, which is eventually converted to fibrin. Most blood tests are done on serum, and therefore require use of a tube that allows blood to clot. Red-top tubes contain no additives; they are used for chemistries, drug monitoring, radioimmunoassays, serology, and blood typing. Lavender-top tubes, which contain ethylenediaminetetraacetic acid (EDTA), are usually used for hematology and certain other chemistries. Greentop tubes contain heparin as the anticoagulant and can be used for chemistries, arterial blood gases, hormone levels, and some immune function studies. Blue-top tubes, used for coagulation studies, contain citrate. Sodium fluoride, found in gray-top tubes, prevents glycolysis and may be used to test blood glucose in its in vivo state.2
When multiple blood samples are drawn at the same collection time, the preferred order is as follows: blood culture tubes, tubes with no preservative (red-top); tubes with mild anticoagulants (blue then green); tubes with EDTA (lavender-top); and oxalate/fluoride tubes (gray-top) should be collected last. Blood for coagulation studies should never be drawn first because tissue injury can initiate the clotting process and result in falsely low levels of coagulation factors. Specimens in tubes with additives should be rotated gently to mix the anticoagulant with the blood and should never be shaken.2
Hemolysis refers to the lysis of red blood cells (RBCs). When extracellular fluid (plasma) is used for analysis, inaccurate results are produced if the specimen is hemolyzed. Hemolysis may occur in vivo, as in hemolytic disease states such as transfusion reactions. Hemolysis may also occur in some infections and with the use of some drugs. A deficiency of the enzyme glucose-6-phosphate dehydrogenase, responsible for generating chemicals needed for maintenance of normal red cell fragility, contributes to hemolysis.
Hemolysis may also occur as a result of improper specimen collection technique or specimen transport. Hemolysis is the cause for specimen rejection in most nonemergent situations. Specimens may be hemolyzed if they are collected from a poorly flowing venipuncture. The selection of the appropriate size needle and catheter is essential when performing venipuncture. Failure to dry alcohol from the venipuncture site also results in hemolysis. Blood should never be forcibly withdrawn from the venipuncture, nor should it be forcibly entered into the collection tube by pushing on the syringe barrel to fill faster. Specimens should be handled carefully when placed in collection tubes and when transported to the laboratory; rough handling may lead to hemolysis.
Hemolysis increases the laboratory values of creatine kinase (CK), potassium, magnesium, calcium, and phosphorus.2
Hemolysis invalidates the results of most coagulation tests and can mask hemolyzing antibodies in the antibody screen and crossmatch.2
If unexpected elevated laboratory values are reported, the blood should be redrawn if hemolysis is suspected.
Nursing research continues to evaluate different techniques and equipment in determining the best method to withdraw blood, obtain accurate results, and reduce hemolysis. Controversy
still exists as to whether aspirating blood from an IV catheter or saline lock provides less hemolysis than venipuncture with a needle. Dugan et al.4
found drawing blood from a size 22-gauge IV catheter caused the most hemolysis in their emergency department. Lowe et al.5
found that venipuncture had less hemolysis than IV catheters, Kennedy et al.7
found larger size IV catheters caused less hemolysis than smaller sized catheters, and Cox et al.8
found using a 5-ml vacuum collection tube demonstrated better results than 10-ml vacuum tubes. However, Corbo et al.9
determined aspirating the sample from a saline lock after discarding blood did not result in more hemolysis than venipuncture. Arrants et al.11
found similar results using 18-gauge saline locks for use with coagulation studies. These studies demonstrate how important technique is to specimen collection.
Proper specimen collection includes accurate identification of the patient and accurate labeling of the specimen at the site of collection. It also includes rapid transport to the laboratory, because cells remain viable after collection and continue their metabolic processes. Specimens that are left to stand unprocessed often yield inaccurate results.
Interpretation of Results
Inherent physiologic variability exists based on patient age, sex, ethnicity, and health status (such as, pregnancy or post-myocardial infarction [MI]). These physiologic differences affect interpretation of results. Physiologic changes associated with the aging process bring concomitant changes in some expected laboratory results. Because men usually have more muscle mass than women, gender differences are seen in substances related to muscle function or metabolism, such as creatinine. There may be significant differences among European, African, and Asian populations in testing for cholesterol, enzymes, and hormones. Various physiologic states, such as pregnancy, stress, obesity, and endurance exercise, also introduce situational changes in expected results.12
Cyclic variability produces daily, monthly, or yearly patterns in physiologic states. These cycles are often taken into consideration in the collection or interpretation of laboratory results.12
As a result, most routine specimens, at least in the hospital setting, are drawn in the early morning to control for any circadian variability.
Blood tests are sometimes affected by the ingestion of food or fluids. Not only are results affected by the absorption of dietary components into the blood after a meal, but hormonal and metabolic changes occur as well. Partial control for the variability introduced by food or fluid ingestion can be achieved either by drawing early morning, pre-meal specimens, or by having the patient fast for 8 to 12 hours. The latter is especially important in lipid testing.13
Sometimes, differences based on position are negligible. In other cases, they are significant. Patient position during (and before) sampling can affect results. In the upright position, there may be a shift in extracellular fluid volume into the tissues. With the resulting increased concentration of proteins and protein-bound substances in the vascular space, samples for proteins, enzymes, hematocrit (Hct), hemoglobin (Hb), calcium, iron, hormones, and several drugs may show an average 5% to 8% increase. Redistribution of extracellular fluid volume and electrolytes within the vascular space does not stabilize until a patient has assumed the sitting position for at least 15 minutes (from a standing position), and in some cases 20 to 30 minutes. In some settings, such as the hospital, it is not difficult to stabilize the patient’s position and thus reduce variability. In other settings, such as ambulatory care, significant variability is introduced if the patient is not made to sit for at least 15 minutes before the blood draw. Because control over sitting time is not usually feasible or practical, care should be taken in the interpretation of results. Exercising immediately before blood sample collection frequently produces significantly erroneous results, especially with enzyme evaluation. Forearm exercises before blood withdrawal may lead to hemolysis.
The timing of blood sampling should include consideration of the effect of medications on the interpretation of results. Medications affect results of many specimens drawn for chemistry, hematology, coagulation, hormonal, and enzyme studies. Knowledge of the effect of the drug assists in proper timing or subsequent interpretation of the results. Consideration should also be given to the effects of other influences, including over-the-counter medications, caffeine, nicotine, ethanol, home remedies, and herbal therapies.
In therapeutic drug monitoring, blood drug levels are monitored to evaluate the effects of drug therapy, make decisions regarding dosage, prevent toxicity, and monitor patient adherence. Timing of the blood sample usually depends on the half-life of the drug; samples drawn at projected peak level assist in monitoring for toxicity, whereas levels drawn at trough help to verify the minimum satisfactory therapeutic level for that patient. Regardless of the purpose of the blood sample, drugs that may affect interpretation of results should be noted on the laboratory slip. For therapeutic drug monitoring, it is important to note the date and time of the last dose as well.
Different laboratories use different equipment and methods by which to test specimens. Specific reference ranges are usually reported alongside the patient’s results on the laboratory report. In an effort to establish a standard for communicating laboratory results, the World Health Organization has recommended that the medical and scientific community throughout the world adopt the use of the International System of Units (ISU). An international unit is defined as the number of moles of substrate converted per second under defined conditions. Thus, many laboratories may report results in different ways, depending on their accepted standard of practice. Most laboratories also report critical (or panic) values. These values should be reported promptly to the provider so that results may be evaluated (and decisions made) in light of the patient condition.
Most reference ranges have been established for venous blood samples. Because arterial blood has higher concentrations of glucose and oxygen and lower concentrations of waste products (i.e., ammonia, potassium, and lactate), an arterial source (instead of venous) should be noted on the laboratory slip. Capillary samples yield results that are closer to arterial blood than venous.
Critical evaluation of laboratory results should take into account how the reference or “normal” values were determined. Patients who have been seen for a long time by the same provider, or those who have been seen within the same health care organization, sometimes establish their own reference range. Reference ranges for a specific disease are sometimes established through large-scale clinical trials.
In most circumstances, each laboratory establishes its own reference values by testing a group that is easy to recruit. It is possible, however, that this technique may not reflect the usual values or range of values of the group that the organization serves. When samples are taken from volunteers, such as those who agree to give a blood sample for reference testing in exchange for a free cholesterol screening, bias may be introduced because those who are likely to volunteer may be those who have or suspect they have illness already. When reference samples are taken from patients who are undergoing routine physical examinations or elective surgery, results may reflect a mix of the surrounding population. Again, these reference values need to be considered in light of who was included or excluded from testing. Usually, those who drink alcohol, smoke, or take certain medications are excluded from reference range testing. However, this exclusion is likely to establish a narrow range of “normal” values, thereby increasing the number of people in the served population who fall outside the established range. Additional care should be taken in interpreting results if the laboratory reports only one set of reference values.
Clinicians who are aware of how reference ranges are obtained are in a better position to interpret laboratory results accurately for their patients. In all situations, interpretation of results should be done in light of all factors that introduce variability, and in light of the clinical condition, remembering that “normal” values do not necessarily indicate absence of disease; just as “abnormal” values do not necessarily establish a pathologic state.
Point-of-care testing (POCT) also known as Bedside Testing or Alternative Site Testing, is the laboratory testing of blood that is performed outside of a central laboratory. The goal of POCT is to reduce the time it takes to diagnose and treat the patient (decision cycle time). Since laboratory analysis of blood comprises approximately 43% of the data used by health care workers to make clinical decisions,1
POCT provides a decrease in the number of steps required to obtain a blood sample, process the sample, and receive the data, and therefore reduces decision cycle time. POCT is ideal in intensive care units, emergency departments, cardiac catheterization laboratories, and surgical suites where the need for rapid turnaround time of laboratory data is desired. Benefits of POCT include decreased turnaround time, improved patient management, increased patient satisfaction, improved job satisfaction of nurses and physicians, decreased operating room time, decreased mortality and morbidity, and less blood sample volume.
Glucose monitoring has been available for years as POCT to guide dosage of insulin administration. Hospitals have also used portable activated clotting time (ACT) monitors to guide anticoagulation and heparin administration during interventional cardiology procedures and during cardiovascular surgery. In addition to glucose and ACT, POCT assays that are available for care of cardiac patients include Hct, Hb, arterial blood gases (ABGs), electrolytes, blood urea nitrogen (BUN), creatinine, ethanol, drugs of abuse, troponin-I, troponin-T, myoglobin, CK-MB, and Type-B natriuretic peptide (BNP). Use of POCT cardiac biochemical marker testing has increased from 4% in 2001 to 12% in 2004 and is expected to rapidly expand.16
To ensure accuracy of data, a POCT system requires that there be up front training of non-laboratory personnel on how to use new equipment, continued proficiency testing of staff, and assurance that electronic quality control requirements are met. It is important that POCT systems are linked to hospital or laboratory systems by radiofrequency and infrared to ensure that information handling, storage, and billing are done properly.
Possible limitations of using a POCT system include its use by personnel with limited training in laboratory technology and the lack of understanding of quality control. POCT is considered to be more expensive than traditional laboratory analysis because the cost of cartridges is more expensive. Cost analysis needs to include the decreased labor by nursing and laboratory personnel plus the ability to make rapid decisions about acutely ill patients that may alter their course of illness.1
Administration of a POCT system includes designating someone to be responsible for the POCT service, which would include: knowing who is performing POCT and which test they are performing, maintaining quality control documentation, selecting appropriate equipment, troubleshooting all aspects of POCT, coordinating training, and serving as a liaison between nursing and other services.
Ng et al.17
and Singer et al.18
studied use of POCT of cardiac biomarkers in the triaging of patients with chest pain. Cardiac marker POCT reduced length of stay in the emergency department18
and allowed for accurate triaging of chest pain patients within 90 minutes of presentation to the emergency department.17