This article was adapted from AACC's Clinical Laboratory News. For the full article see: (May 1, 2016) Khushbu Patel, PhD, Sarah M. Brown, PhD, and Dennis J. Dietzen, PhD. Listening Closely when the Volume is Turned Down, Challenges to Small Volume Testing. Available online at https://www.aacc.org/publications/cln/articles/2016/may/listening-closely-when-the-volume-is-turned-down
Collecting Smaller Blood Samples for Testing May Benefit Patients but Pose Challenges for Labs
In 1999, Jocelyn M. Hicks, PhD, a laboratory director who was hospitalized for several months, developed anemia in the hospital and required several blood transfusions. After she recovered, she was shocked to discover that the anemia was caused by the amount of blood drawn for her lab tests during her admission. This prompted her to survey 19 community, university, and children's hospitals about blood-drawing practices. Hicks discovered that most respondents collected 10 times the volume of blood necessary for routine laboratory testing. About one-half to two teaspoons (2.5–10 mL) of blood was collected for testing that required just one-third teaspoon (1.5 mL) or less.
Little has changed over the ensuing 17 years, despite the fact that modern chemistry analyzers require less than 50 microliters (about a drop) of plasma for a routine comprehensive metabolic panel. Recent publications have shown a direct relationship between patients' ill health and the volume of blood collected for laboratory testing.
Given these findings, it is not surprising that blood-drawing (phlebotomy) has been implicated as a primary cause of anemia occurring during hospital stays (hospital-acquired anemia, HAA). Anemia is characterized by a low number of red blood cells and low hemoglobin and can cause fatigue and weakness. One study found that the rate of hospital-acquired anemia was approximately 45% in patients older than 18 years admitted to an internal medicine inpatient service. Another study of 30 pediatric intensive care units (PICUs) reported a similar rate of 41%. Hospital-acquired anemia also has been associated with higher rates of in-hospital illness and death.
Due to these concerns, there is a constant push to reduce the amount of blood collected in pediatric patients as well as in adults, and in hospitals as well as in out-patient care settings. However, while collecting the very minimal blood volume required for testing reduces the risk of anemia, laboratory testing on smaller blood volumes poses several challenges for clinical laboratories. Here are just a few examples:
Minimum sample required for automated instruments:
- The minimum amount of blood needed to perform tests on an automated instrument depends on the total amount needed for the individual tests plus the amount necessary for the instrument to accurately pick up or aspirate a sample (often referred to as the "dead volume"). The dead volume can be significant depending on the test and instrument. For example, a test might require less than a drop (30 microliters) of sample, but an instrument's dead volume might be 10 times greater.
- Some instruments test for substances in samples that can interfere with testing, such as the presence of hemoglobin from red blood cells that have broken apart (hemolysis). The tests for interferences are usually performed prior to the actual test on chemistry instruments and can add to the total volume required.
Sample collection tubes and additives:
- Many sample collection tubes contain additives that are needed for testing to be performed accurately. If a smaller amount of blood is put into a collection tube containing a fixed amount of additive (anti-coagulant), it will lead to decreased blood-to-additive ratio. This can cause inaccurate test results. For example, the tubes used to collect blood for a prothrombin time (PT) contain sodium citrate. If citrate tubes are under-filled with smaller blood samples, the abnormally high concentration of citrate in the tube can cause results to be abnormal (falsely prolonged clotting times).
- Because certain tests can only be performed when the blood is collected with a specific additive, various tubes of blood are often obtained, each of which must contain a certain amount to ensure testing accuracy.
- Small volume samples can be collected in smaller tubes or in microcollection tubes often referred to as "bullets" (Photo source: Khushbu Patel, PhD). These tubes are designed for smaller volumes and contain lower amounts of additives. However, these bullets may not fit all automated instruments and present several other challenges that laboratories must manage.
Capillary blood samples:
- Small blood samples are often obtained from a finger stick or heel stick (capillary blood collection). Capillary blood collections historically have been used in children, but this practice is increasingly attractive in adult care. Elderly patients often have poor veins, making for a difficult blood draw from a vein and thus requiring a finger stick. Patients who are undergoing certain treatments (such as chemotherapy) may require regular blood draws or need to conserve their veins for IV access and may also opt for this procedure.
However, using capillary samples for testing may lead to some inaccuracy and results that can vary compared to samples collected from a vein. For example, research has demonstrated that glucose in capillary blood is generally higher compared to blood drawn from a vein.
- Capillary samples are also associated with high rates of red blood cells breaking apart (hemolysis) and clotting. Proper collection technique is essential to minimize hemolysis of a sample from a finger or heel stick. "Milking" or squeezing a finger or heel is discouraged because it can result in hemolysis and can trigger clot formation. Therefore, some lab testing that evaluates a patient’s clotting system (coagulation testing, such as a partial thromboplastin time or PTT) is not recommended on capillary specimens.
Small volume blood samples introduce several challenges for laboratory testing processes. Nevertheless, given the benefits of reducing the impact of blood-drawing on patients' health, labs and lab technology must take steps to meet the unique requirements of small sample sizes. In particular, new instruments designed to accommodate small sample sizes will be critical in overcoming challenges associated with routine use of small-volume samples.