Prehospital blood glucose analysis
A variety of medical conditions and patient presentations warrant prehospital blood glucose analysis
By Kenny Navarro
Prehospital glucometry is a safe, effective and minimally invasive procedure utilized by EMS professionals around the country. Detecting blood glucose anomalies allows field crews the ability to provide early and condition-specific treatment while assisting in triage decisions that make better use of limited resources (Carter, Keane, & Dreyer, 2002; Holstein, Plashcke, Vogel, & Egberts, 2003; Lerner, Billittier, Lance, Janicke, & Teuscher, 2003).
A variety of medical conditions and patient presentations warrant prehospital blood glucose analysis. An altered mental status is the most common adult chief complaint that triggers a blood glucose measurement by EMS personnel (Strote, Cloyd, Rea, & Eisenberg, 2003).
For pediatric patients however, EMS personnel use seizure as the common trigger for performing prehospital glucometry (Vilke, Castillo, Ray, Murrin, & Chan, 2005).
Modern self-monitoring blood glucose (SMBG) monitors, like the ones used by EMS agencies around the country use an enzyme reaction to generate a current, which is measured by the meter (Beardsall, 2010). A higher concentration of glucose in the sample will generate a stronger current and yield a high number on the display.
Each test strip within an individual package is slightly different from every other test strip, however this difference does not generally produce clinically significant results. On the other hand, each package of test strips may come from different batches, which may have variations in the concentrations of the active and inert reagents.
The differences between batches could result in a significant under- or over-estimation of blood glucose levels, potentially causing the patient to under- or overdose insulin. To improve the accuracy of the SMBG monitor, the manufacturer recommends device calibration each time with each new package of test strips.
Calibration can be as simple as entering a code found on the test strip package or inserting a calibration strip before obtaining the blood sample. Newer generations of SMBG monitors use proprietary technology that does not require coding with each new batch of test strips (Alva, 2008; Young, Ellison, & Marshall, 2008).
Despite these advancements in technology, the Food and Drug Administration standards allow all SMBG monitors to have a certain measurement margin of error (Diabetes Forecast, 2013). For example, the standard allows a 20% maximum margin of error in no more than 95% of the cases when the reading is over 75 mg/dL.
In practical terms, when you obtain a blood glucose reading of 100 mg/dL, 95% of the time your patient’s actual blood glucose level will be anywhere between 80 mg/dL - 120 mg/dL. Additionally, five percent of your patients will have an actual blood glucose reading that falls outside of that range.
For blood glucose readings below 75 mg/dL, the maximum margin of error is tighter. At this level, the standard requires that monitor manufacturers demonstrate a 15% maximum margin of error in 95% of the cases.
For device manufacturers to meet this standard, they only need to compare their device against hospital-grade glucose measurement with collection under ideal and error free conditions.
Compliance is voluntary, but manufacturers who choose not to test their devices to this standard must provide documentation to the FDA detailing how their device is as safe as the devices that do meet the accuracy standard (McCarren, n.d.).
In the field, a number of environmental and user factors may increase the margin of error of these SMBG monitors (Accu-Check Connect, n.d.). For example, temperature and humidity may affect the accuracy, with some devices performing better at lower temperatures and some at higher temperature.
Elevated altitude may overestimate blood glucose readings (e Mol et al., 2010). User error may result from poor or missing device calibration, outdated strips, improper technique, poor timing, insufficient sample size, and contamination. Contamination is especially serious since it can happen so easily and is likely to result in episodes of hypoglycemia going unrecognized and untreated. Even a very tiny amount of glucose contamination can seriously alter a reading.
In a group of healthy patients, Daves et al. (2011) demonstrated a strong but unpredictable bias in blood glucose measurement with SMBG monitors caused by abnormal hematocrit levels. The measurement error appears greatest when hematocrit levels are elevated above normal.
Some SMBG monitors simultaneously measure hematocrit levels and reports a blood glucose level corrected for hematocrit (Rao, Jakubiak, Sidwell, Winkelman, & Snyder, 2005).
By taking a few precautions, EMS personnel may increase the accuracy of the glucose measurement. First, always use test strips recommended by the glucometer manufacturer. It is unclear whether glucose readings are accurate if obtained with generic test strips.
In side-by-side comparisons against a known sample, researchers have demonstrated that different test strips will give noticeable variations in blood glucose values (Lenhard, DeCherney, Maser, Patten, & Kubik, 1995).
There are two primary types of SMBG monitors used by EMS systems. One type is calibrated to analyze whole blood sample taken directly from a vein. The far more common type is calibrated for a whole blood sample taken at the capillary level, such as from a fingerstick.
If venous samples are used directly from the IV site for instance, you may get a reading that varies slightly from the true sample (Kumar, Sng, & Kumar, 2004). In healthy volunteers, a poor correlation has been demonstrated between glucometer values using venous samples and values obtained from fingerstick blood (Funk, Chan, Lutz, & Verdile, 2001).
For the most accurate reading possible, you should obtain your blood sample from a capillary source rather than directly from a vein.
Historically and presumably for convenience, medical personnel obtained the capillary sample from the tip of one of the patient’s finger. However, forearm sampling can provide a less painful alternative to fingerstick sampling with acceptable accuracy results (Greenhalgh, Bradshaw, Hall, & Price, 2004).
Regardless of where you intend to obtain the sample, you must first swab the site with alcohol and wipe the area dry with a sterile gauze pad. This allows any residual alcohol to be removed and avoids contamination of the sample blood.
For added accuracy, the first drop of blood available from the fingerstick should also be wiped with the gauze pad. This ensures that the second blood drop is the most accurate and least contaminated sample that can be taken.
Finally, most glucometers are not calibrated for use in neonates, who are generally defined as babies in the first 28 days of life. If blood glucose analysis is performed in neonates, the accuracy is questionable (Beardsall, 2010).
Term neonates may have initial hematocrit levels as high as 60% to 70% (Jopling, Henry, Wiedmeier, & Christensen, 2009). Elevated hematocrit levels may underestimate glucose levels resulting in untreated hypoglycemia in the neonate (Tang, Lee, Louie, & Kost, 2000).
Although the exact incidence of hypoglycemia in the newborn is unknown, one case-controlled study of term delivery from non-diabetic mothers placed the incidence of hypoglycemia in the neonate at about 2.4% (Straussman & Levitsky, 2010).
Hypoglycemia presents with a variety of symptoms and there is little correlation between a specific symptom and a specific blood glucose level (Tintinalli, Ruiz, & Krome, 1996). EMS personnel must have a high index of suspicion for hypoglycemia when assessing any patient with an alteration in mental status.
Despite the limitations inherent in prehospital glucometry, most devices provide a safe and effective method for estimating blood glucose levels in patients.
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