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248 P A R T III / Assessment of Heart Disease
In most circumstances, each laboratory establishes its own ref- specific tests are used to confirm or exclude a suspected disease
erence values by testing a group that is easy to recruit. It is possi- and minimize the number of false-positive results. 15
ble, however, that this technique may not reflect the usual values
or range of values of the group that the organization serves. Point-of-Care Testing
When samples are taken from volunteers, such as those who
agree to give a blood sample for reference testing in exchange for Point-of-care testing (POCT) also known as Bedside Testing or
a free cholesterol screening, bias may be introduced because those Alternative Site Testing, is the laboratory testing of blood that is
who are likely to volunteer may be those who have or suspect performed outside of a central laboratory. The goal of POCT is
they have illness already. When reference samples are taken from to reduce the time it takes to diagnose and treat the patient (de-
patients who are undergoing routine physical examinations or cision cycle time). Since laboratory analysis of blood comprises
elective surgery, results may reflect a mix of the surrounding pop- approximately 43% of the data used by health care workers to
1
ulation. Again, these reference values need to be considered in make clinical decisions, POCT provides a decrease in the num-
light of who was included or excluded from testing. Usually, ber of steps required to obtain a blood sample, process the sample,
those who drink alcohol, smoke, or take certain medications are and receive the data, and therefore reduces decision cycle time.
excluded from reference range testing. However, this exclusion is POCT is ideal in intensive care units, emergency departments, car-
likely to establish a narrow range of “normal” values, thereby in- diac catheterization laboratories, and surgical suites where the need
creasing the number of people in the served population who fall for rapid turnaround time of laboratory data is desired. Benefits of
outside the established range. Additional care should be taken in POCT include decreased turnaround time, improved patient man-
interpreting results if the laboratory reports only one set of refer- agement, increased patient satisfaction, improved job satisfaction
ence values. of nurses and physicians, decreased operating room time, decreased
Clinicians who are aware of how reference ranges are ob- mortality and morbidity, and less blood sample volume.
tained are in a better position to interpret laboratory results ac- Glucose monitoring has been available for years as POCT to
curately for their patients. In all situations, interpretation of re- guide dosage of insulin administration. Hospitals have also used
sults should be done in light of all factors that introduce portable activated clotting time (ACT) monitors to guide anticoag-
variability, and in light of the clinical condition, remembering ulation and heparin administration during interventional cardiol-
that “normal” values do not necessarily indicate absence of dis- ogy procedures and during cardiovascular surgery. In addition to
ease; just as “abnormal” values do not necessarily establish a glucose and ACT, POCT assays that are available for care of cardiac
pathologic state. 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 natri-
Sensitivity and Specificity uretic peptide (BNP). Use of POCT cardiac biochemical marker
of Laboratory Tests testing has increased from 4% in 2001 to 12% in 2004 and is
expected to rapidly expand. 16
Clinicians should use measures of test performance to judge the To ensure accuracy of data, a POCT system requires that there
quality of a diagnostic test for a particular disease. The ability of a be up front training of non-laboratory personnel on how to use
laboratory test to identify a particular disease is quantified by two new equipment, continued proficiency testing of staff, and assur-
measurements: sensitivity and specificity. 14 ance that electronic quality control requirements are met. It is im-
Sensitivity is the frequency of a positive (abnormal) test re- portant that POCT systems are linked to hospital or laboratory
sult among all patients with a particular disease or the likelihood systems by radiofrequency and infrared to ensure that information
that a diseased patient has a positive test. If all patients with a handling, storage, and billing are done properly.
given disease have a positive test, the test sensitivity is 100%. Possible limitations of using a POCT system include its use by
Sensitivity is calculated by testing a population of patients who personnel with limited training in laboratory technology and the
have been found to have a particular disease by some “gold stan- lack of understanding of quality control. POCT is considered to
dard” method (a procedure that defines the true disease state of be more expensive than traditional laboratory analysis because the
the patient). 14 cost of cartridges is more expensive. Cost analysis needs to include
Specificity is the frequency of a negative (normal) test among the decreased labor by nursing and laboratory personnel plus the
all persons who do not have the disease or the likelihood that a ability to make rapid decisions about acutely ill patients that may
healthy patient has a negative test. If all patients who do not have alter their course of illness. 1
a particular disease have a negative test, the test specificity is Administration of a POCT system includes designating some-
100%. A test with a high specificity is helpful to confirm a diag- one to be responsible for the POCT service, which would include:
nosis, because a highly specific test will have few results that are knowing who is performing POCT and which test they are per-
falsely positive. Specificity is calculated by testing a population of forming, maintaining quality control documentation, selecting
patients who have been found to have a particular disease by some appropriate equipment, troubleshooting all aspects of POCT, co-
gold standard method. 14 ordinating training, and serving as a liaison between nursing and
Under the best of circumstances, no blood test is perfect and other services.
results may be misleading. Sensitivity and specificity may be al- Ng et al. 17 and Singer et al. 18 studied use of POCT of cardiac
tered by the coexistence of other diseases or complications from biomarkers in the triaging of patients with chest pain. Cardiac
the primary disease. The most sensitive tests are used to rule out a marker POCT reduced length of stay in the emergency depart-
suspected disease so that the number of false-negative tests is min- ment 18 and allowed for accurate triaging of chest pain patients
imal; thus, a negative test tends to exclude the disease. The most within 90 minutes of presentation to the emergency department. 17