Comparison of Warmed and Un-warmed Heel Stick Capillary Blood Glucose Samples in Neonates

Neonatal hypoglycemia is defined by the American Academy of Pediatrics as blood glucose level less than 47 mg/dL and is one of the most common diagnoses requiring admission to the neonatal intensive care unit (NICU).1,2 The incidence of hypoglycemia in healthy newborns is approximately 5-15%, and as high as 51% in infants with risk factors.3 The cost of hospitalization of neonatal hypoglycemia is estimated to range from $2,350-$12,755.4 Hypoglycemia in the newborn has been associated with permanent brain injury including neurodevelopmental delay, learning difficulties and seizures.5 Clinically important hypoglycemia is not always associated with clinical exam findings;6 therefore, accurate measurement of blood glucose in the newborn is of great importance.

Laboratory testing of venous blood, typically plasma, is a widely accepted method of diagnosing neonatal hypoglycemia.6 However, point of care (POC) testing of capillary blood using a bedside glucometer is a common screening tool in newborn nurseries and NICUs. The POC testing method provides real time results and is an easier, less invasive sampling technique.6 Despite these conveniences, studies have found POC testing to be less accurate than venous testing.7-9 POC testing is also less accurate at lower glucose levels and readings are affected by polycythemia and hyperbilirubinemia, common findings in newborn babies.6,7,10 Inaccurate readings may result in unnecessary admissions to the NICU for additional testing and management. Therefore, efforts to optimize the accuracy POC glucose testing in screening for neonatal hypoglycemia is imperative.

One common practice thought to increase the accuracy of capillary POC glucose testing is to warm the infant's heel before blood sampling. It has been proposed that warming the heel will increase blood flow to the capillary bed and improve peripheral blood stasis that may otherwise result in a falsely low glucose reading.11,12 Current practice is to warm the heel with a gel heat pack designed and marketed specifically for the purpose of warming the infant's heel prior to blood collection. These gel heat packs are an additional cost to the hospital, and currently it is unclear if there is any significant clinical benefit to warming the infant's heel before obtaining capillary blood glucose measurements.

Various studies utilizing laser Doppler flowmetry have suggested that local warming results in increased skin blood flow and perfusion in infants.13-15 This increase in blood flow is thought to be reflected in the volume of blood collected and ease of collection. In practice, however, the effect of local warming on capillary blood collection in neonates appears to be minimal. One study found that heel warming in infants did not improve the volume of blood collected or the ease of blood collection.16 A subsequent study found that warming the heel decreased the need to squeeze the heel for blood collection, however, there was no difference in volume of blood collected or collection time.17 To the best of our knowledge, the only study that has directly compared the effect of warming the heel on capillary blood glucose in infants was conducted by Russell and McKay in 1966.18 The study compared simultaneously collected capillary blood glucose levels from a warmed and un-warmed heel in 57 infants. Overall, glucose levels from the warmed and un-warmed heels were highly significantly correlated. However, the mean glucose concentration from blood in warmed heels was 2.8 mg/dL higher than that from un-warmed heels. Additionally, of the 57 infants, 13 had a statistically significant higher glucose level measured in their warmed heel compared to their un-warmed heel.

The Russell and McKay study suggested that the infant's heel must always be warmed before collecting capillary blood for glucose measurement. However, the study included only 57 infants, a subset of which were also assigned to a low calorie, high calorie, or hydrocortisone administration groups to determine the outcomes of these interventions on blood glucose levels. Additionally, the infant's heels were warmed by soaking in warm water. This is no longer the current practice as most institutions use gel heat packs designed specifically for warming the heels of infants.