Point-of-Care testing (POCT) provides rapid test results with the potential to improve treatment. Additional advantages of POCT include small sample volumes, a wide variety of tests available, little to no processing required to run the tests, and ease of use within the clinical patient flow. However, when incorrectly performed or inappropriately utilized, POCT can generate misleading results that require additional follow-up testing at increased cost and risk to the patient.
Just as in traditional laboratory testing, the majority of POCT errors occur in the pre-analytical phase. These are processes that occur before the specimen is analyzed. Unfortunately, the instrument, operator, or clinician interpreting the results cannot readily identify most pre-analytical errors. Some pre-analytical variables specific to POCT are patient misidentification, improper specimen collection, air bubbles, hemolysis, improper site selection, and interfering substances.
Common Errors in POCT
Patient misidentification is a major source of error in POCT. Many POC devices use manual entry for patient identification, which leads to typos and incorrect information being entered. If a POC device is equipped with a barcode scanner, it is recommended to utilize the scanner to prevent such errors. For samples that must be collected from the patient and transported to the POC device, the small size of the sample container can make properly labeling the specimens a challenge. However, it is imperative that two patient identifiers are verified prior to performing testing.
Another common error is clinicians scanning loose labels in a patient’s room. It is not unusual to find other patient labels in a room that may have been left by a previous patient or accidentally carried into the room by the clinician. It is important to always scan the patient’s armband that is worn by the patient to assure accurate identification prior to testing.
Specimen collection is also a source of error when performing capillary collections for POCT. These errors can include poor lancing technique, using a previously punctured site, milking the puncture site, not producing an adequate volume of blood prior to adding to the test strip or cuvette, and air bubbles in the sample. Another problem: performing a finger stick on the same side as a peripheral IV infusing a substance that may interfere with the assays, such as dextrose or insulin.
Milking refers to excessive squeezing or massaging and is generally done when not enough blood volume is being produced. Milking is often the result of poor lancing technique and can cause decreased concentrations of some analytes due to dilution of the blood sample with tissue fluid. To avoid milking a puncture site, it can be helpful to use a lancet of adequate size and to warm the collection site to increase blood flow prior to puncture.
Air bubbles may cause erroneous results, particularly in blood gas measurements for pCO2 and pO2. Air bubbles present in cuvettes for hemoglobin tests that use optical readings can also cause erroneous results. It is vital that blood gas samples and cuvettes are collected free of air bubbles to obtain accurate results. Air bubbles in blood gas samples can be prevented by collecting the first drop of blood in one fluid motion. If a second drop is needed, the capillary tube should remain at an upward angle while the second drop is being formed to prevent air from entering the tube.
Samples with hemolysis can also impact results for certain analytes. Hemolysis can be caused by different factors, but a large contributor is milking of the puncture site. Analytes that are affected by hemolysis include potassium, aspartate aminotransferase, and lactate dehydrogenase. The results of these analytes will increase as the degree of hemolysis increases. Ways to prevent hemolysis are to avoid milking the puncture site, ensuring the puncture site is completely dry if using alcohol to clean, and selecting the appropriate size of lancet for the volume of blood needed. Unfortunately, there are no current methods to detect hemolysis in the POCT setting, and it is likely to be missed.
Selecting the proper location to perform a capillary puncture is important. Clinicians should avoid sites that have been previously punctured and perform proper cleaning. Our pediatric institution only allows capillary collections to be performed on a patient’s heel, middle finger, or ring finger, though ear lobes and other sites are also used for capillary punctures at other institutions.
Lastly, interferences are another source of pre-analytical errors that can impact POCT. For example, glucose is one of the most highly utilized POC tests, and it is very important for patient monitoring along with insulin management. However, POC glucose can be affected by interference of ascorbic acid (vitamin C) and body lotions containing hydroquinone. Ascorbic acid interferes with glucose measurements on some meters due to the method the meter uses. Depending on the type of meter and concentration of ascorbic acid in circulation, the results can be falsely increased or falsely decreased.
Hydroquinone-containing body lotions can falsely increase glucose measurements. These lotions are commonly used to reduce age spots and lighten dark spots on the skin. It is important that both the practitioner and patient be educated about possible interferences so proper testing technique is used to ensure reliable results.
The request for POCT continues to grow. POCT has unique challenges because it is not typically performed by lab personnel. Training and continuous education for clinicians is important to help reduce errors. A quick result does not always mean an accurate result.
Jessica Jenkins, MSA, MLS (ASCP) CM, is manager of point of care testing at Nationwide Children’s Hospital in Columbus, Ohio. +Email: [email protected]