Warming Prior to Heel Stick

Abstract

Background:

Capillary blood sampling (heel stick) in infants is commonly performed in neonatal care units. Before the procedure, warming the infant's heel is often a customary practice, but no consensus exists on the most effective heel-warming method.

Purpose:

To compare the effects of routinely used warming methods (glove, gel pack, or blanket) applied prior to heel stick on blood sample quality and infant's comfort.

Methods:

This prospective, double-blind, randomized controlled trial conducted in the neonatal intensive care unit included infants (postmenstrual age of ≥28 + 0 weeks and ≤43 + 6 weeks) who were computer-randomized to 1 of 3 warming methods.

The primary outcome was blood flow velocity at sampling. Secondary outcomes were hemolysis index, infant COMFORTneo score, and frequency of postprocedure skin injuries. In addition, irrespective of the warming method used, the correlation between heel skin temperature and postprocedure heel skin injury was analyzed.

Results:

A total of 176 heel warmings were successfully randomized, and 173 were analyzed. Despite a significant difference in obtained heel skin temperature after warming between the 3 warming methods (P = .001), no difference in blood flow velocity (P = .91), hemolysis index (P = .99), or COMFORTneo score (P = .76) was found. Baseline skin temperatures above 37.0°C were associated with higher incidences of skin injury, and skin temperatures after warming were significantly higher in skin-injured heels (P = .038).

Implications for Practice and Research:

All 3 warming methods had similar effects on blood sample quality and infant's comfort. However, excessive warming of the heel should be avoided to prevent skin injuries.

BACKGROUND AND SIGNIFICANCE

Preterm and sick newborn infants are admitted to neonatal intensive care units (NICU) for specialized treatment and care, which may include potentially painful clinical and diagnostic procedures such as capillary blood sampling (heel stick).1 Repeated painful procedures may not only have a negative impact on the acute physiological parameters of the infant but also have the potential to cause long-term neurodevelopmental disabilities with major consequences for both cognitive and emotional development.2–5 A study from 2019 reported 13.9 painful procedures per day per infant admitted to the NICU with heel stick being one of the most frequent painful procedures.6 Therefore, reducing the number of painful procedures as well as the pain experienced during the necessary procedures must be considered as an important task for healthcare professionals.1–3 To reduce or relieve the infant's pain during heel stick procedure, various methods are recommended, such as sucrose analgesia, lactation, skin-to-skin care, vibration, swaddling, and warming the infant's heel before the procedure.3,7–10

Infant's heel warming aims to minimize the risk of difficult blood sampling by increasing the capillary blood flow.11 A better blood flow at the incision site is presumed to decrease the duration of the sampling procedure and reduce the pain and discomfort for the infant.7,9 The sampling duration is therefore an important procedure quality estimate. Furthermore, a high blood flow is presumed to lower the need to exert pressure on the incision site, such as by squeezing, thereby minimizing the risk of hemolysis.11,12 Hemolysis, resulting from inappropriate or incorrect handling of procedures during blood collection, is the breakdown of red blood cells and is recognized as a major preanalytical problem because a range of biochemical tests are unmeasurable in this situation.12–14 The hemolysis index aids in evaluating sample integrity as an indicator of the number of destroyed red blood cells present in a sample.13,15 Warming of the incision site before heel stick may be beneficial for collecting pH and blood gas specimens as well,16,17 and studies also found that warming the heel may reduce the infant's pain.7,18

The purpose of heel warming is to ensure the quality of both the procedure and the sample obtained. However, there is no consensus regarding how heel warming should be performed.

The World Health Organization recommends keeping the infant's body warm to facilitate the procedure by swaddling the infant in a blanket or putting a warm cloth on the incision site.11

The 2020 Clinical and Laboratory Standards Institute (CLSI) guidelines are not particularly clear, describing incision-site warming as not generally recommended in the current literature, yet, recommending that the incision site have an adequate temperature before heel stick and advising that commercial heaters should be used in accordance with the manufacturer's instructions.17 They also stated that warm towels can be used for the purpose of heel warming as long as the temperature does not exceed 42°C.17 Others have suggested using a thermal bag,18 dry compression,7 and the warmth of hands9 for heel warming. In Danish NICUs, use of heated blankets, heated gel packs, and gloves with hot tap water are the most frequently used warming methods.

For some years, the neonatal nurses within the NICU at the Department of Pediatrics and Adolescent Medicine, Lillebaelt Hospital, University Hospital of Southern Denmark, Kolding (NICU Kolding), have used a glove filled with hot tap water, assessed to be hot enough to warm the heel without the risk of burning the heel skin, as the warming method prior to heel sticks. However, the department's quality-monitoring program has revealed occasional inadvertent events, such as severe bruising and burns following heel warming and heel sticks (unpublished data).

The current randomized controlled trial aims to compare 3 different routinely used warming methods (glove, gel pack, or blanket) applied prior to infants' heel stick on blood sample quality and infant's comfort with the purpose of improving clinical care for these patients while acknowledging the variety of recommendations regarding warming methods.

What This Study Adds

• Provides healthcare professionals new information about how warming methods before heel stick influence the infant's procedural comfort.

• Reveals how skin temperature can be related to skin injuries due to heel stick.

• Reflects upon the effect of warming the heels before heel stick in general.

• Recommends the use of blood flow velocity instead of procedure time as a more appropriate measurement for procedural quality.

• Puts a new perspective on warming methods by taking into consideration the preanalytical quality of the blood sample.

• Suggests future research on different warming methods in regard to the benefits and harms of these.

METHODS

Design and Sample

This prospective double-blind randomized controlled trial was carried out during daytime hours (7 am to 3 pm) from August 26, 2020, to February 12, 2021. Heel stick procedures in infants were assessed for eligibility if both parents signed a Danish parental consent form.

Inclusion criteria: Heel sticks performed in infants with a postmenstrual age of 28 + 0 weeks or more and 43 + 6 weeks or less.

Exclusion criteria: Heel sticks in infants with suspected coagulation disorder or severe illness requiring transfer to a specialized hospital.

Infants were randomized to 1 of the 3 warming methods in a mixed combination of 3 and 6 block sizes. Randomization was computer-generated using Research Electronic Data Capture (REDCap) administered by the Open Patient Data Explorative Network, Odense University Hospital, Region of Southern Denmark.19,20 Each infant participated in the study with 1 heel stick per day, with a maximum of 3 in total. Each heel stick that an infant participated in generated a new randomization to one of the warming methods.

Ethical Considerations

Ethical approval was provided by the Region of Southern Denmark (journal no. 20/1316), as well as by the Regional Committees on Health Research Ethics for Southern Denmark (project ID S-20200082). Participation was voluntary, and withdrawal from the study was possible at all times without any consequences for the treatment of the infant. In addition, physicians could terminate or pause participation if the infant's clinical condition deteriorated. All data registration, storage, and statistical analysis were handled confidentially, and participants were pseudonymized after blood sampling. The data were stored in accordance with the protection of personal data.

Setting

The study was carried out at NICU Kolding, a 19-bed level II/III department with an annual admission of approximately 600 critically ill infants as well as premature infants of gestational age of 28 weeks or more, independent of weight. Although neonatal nurses perform capillary blood glucose for point-of-care testing, other blood sampling is routinely performed by biomedical laboratory scientists (BLSs) specifically trained in capillary and venous blood sampling in infants.

Warming Methods

The 3 warming items used were (1) a disposable glove containing water (glove), (2) a Flexxum 15 × 19 cm gel pack (gel pack), and (3) a cotton blanket normally used to swaddle infants (blanket). To warm these items at a controlled temperature, to ensure reproducibility of the study and as a safety measure to avoid burns of the heel, we used an incubator set at 39°C (Memmert IF75m, 74 L, fan 80°C).9,17 A preliminary pilot study showed that the items needed storage in the incubator for minimum 1 hour to gain maximum warmth and show consistency in temperatures.

Outcome

The following outcome measures were defined to determine the differences between the 3 warming methods.

Primary outcome:

• Flow velocity of blood collected during the heel stick procedure (μL/s).

Secondary outcomes:

• Hemolysis index (μmol/L) of the blood sample.

• The infant's COMFORTneo score21 during heel stick procedure.

• The number of heels with skin injuries 2 hours after the heel stick.

Post hoc analysis:

• The correlation between heel skin temperature and the number of heels with skin injuries 2 hours after the heel stick procedure.

Procedure

The infant was placed in an incubator, a cradle, with skin-to-skin contact, or in the arms of a parent. During the procedure, a parent or neonatal nurse comforted the infant with oral sucrose (25%) or human milk to reduce procedural pain, as described by McPherson et al.22 All heel stick procedures were performed by a BLS no earlier than 2 hours after delivery. The principal investigator (PI) ensured both computer-generated randomization and blinding of the warming item for both neonatal nurses and BLSs. The PI wrapped the warming item around both heels of the infant for 5 minutes17 and measured the heel skin temperature before and after warming using an infrared thermometer (Microlife NC 200, Widnau, Switzerland, measurement accuracy ± 0.2°C)23 held approximately 2 cm above the heel. Immediately after the warming was completed, the heel stick was performed according to the standard procedure at NICU Kolding, which is similar to the procedural guidelines of CLSI17 and the World Health Organization.11 In case of disrupted skin integrity such as bruising or swelling, the BLS chose a different incision site. While holding the infant's foot, an automated incision was made in the heel using Tenderfoot from Accriva Diagnostics, and blood was collected in microtainers (Microvette 500) to obtain either EDTA or heparin-stabilized blood for the analysis parameters requested for the treatment of the infant. In some cases, extra blood was collected to ensure that sample material for hemolysis index assessment was available from the whole patient group to avoid selection bias. The extra requisition of blood was performed together with regular blood samples. The total amount of capillary blood collected at each sampling was between 95 and 1345 μL, which, in clinical terms, was considered unproblematic to spare, even for the smallest infant.24,25 The PI attended every heel stick to ensure the consistency of all warming procedures and data collection for the study.

Two hours after the procedure, the heel was examined visually by a neonatal nurse for bruises, swelling, or normal skin (nothing to report), which was registered as the postprocedure heel appearance. No follow-up assessment was conducted. A group of BLS, well-trained to the procedural standard used, performed the heel stick procedures. After the heel stick, the BLSs anonymously recorded answers to open-ended multiple-choice questions in a registration form (see Supplemental Digital Content Appendix 1, available at: http://links.lww.com/ANC/A227) on their years of work experience and their subjective grading of the level of heel squeezing required during the procedure on a scale ranging from 0 (the most squeezing) to 5 (the least squeezing). All the registered data were assembled electronically using REDCap. As an indicator of sample quality, plasma of the heparin-stabilized blood sample was subsequently analyzed for hemolysis index measured in μmol/L on the automated Cobas 8000 chemistry instrument from Roche Diagnostics, Mannheim, Germany.15

Demographic and Physiologic Data

The PI recorded the infant's characteristics, such as gestational age, chronological age (days), birth weight (grams), latest measured weight (grams), and the location of the infant during the procedure (incubator, cradle, skin-to-skin contact, or in the arms of the parent). Furthermore, the applied warming item, heel skin temperature before and after warming, volume of blood collected, incision site (lateral or medial surface of the heel), and number of incisions were recorded (see Supplemental Digital Content Appendix 2, available at: http://links.lww.com/ANC/A228). The BLS and neonatal nurse were blinded to all the data recorded by the PI. The duration of the procedure from the first incision of the heel to the completion of the last microtainer was also recorded. In other studies,7,9 procedure duration was used to estimate procedure quality; however, since duration varies with the amount of blood collected, we considered blood flow velocity to be a superior surrogate outcome measure for procedure quality. Blood flow velocity was calculated by dividing the volume of blood collected (microliters) by the duration of the procedure (seconds).

In a separate data collection form (see Supplemental Digital Content Appendix 3, available at: http://links.lww.com/ANC/A229), the neonatal nurse registered data concerning the infant's comfort 5 minutes before warming of the heel as well as during the last period of blood sampling using the COMFORTneo score, which is an evidence-based pain-scoring system recommended in Denmark for both preterm and full-term infants.26,27 It is measured by observing 6 behavioral dimensions of the infant for 2 minutes: (1) alertness, (2) calmness/agitation, (3a) crying (spontaneously breathing infants only), (3b) respiratory response (mechanically ventilated infants only), (4) facial tension, (5) muscle tone, and (6) body movement. Each dimension is scored from 1 to 5 (5 being the most reactive), and a total score from 6 to 30 is obtained. A total COMFORTneo score of 14 and more indicates that the infant is experiencing pain or discomfort that requires intervention.21,27 All neonatal nurses were certified to determine COMFORTneo scores.

Statistical Analysis

The sample size was estimated as the primary outcome, with a power of 90%. To account for multiple testing among the 3 warming methods, the significance level was lowered to .017. Calculations based on the ability to detect a clinically significant difference of 20% in blood flow velocity resulted in the need for at least 45 heel sticks per warming group. Because of the risk of dropouts, the number of heel sticks was raised by 25% to a sample size of 171, with 57 in each warming group. Continuous data that were not normally distributed were analyzed after log transformation. Because some infants were randomized 2 or 3 times, analyses were performed with linear regression with an adjustment for clusters using STATA release 17.28 Figures were created using R Core Team 2021,29 specifically the R-package Ggstatsplot.30

RESULTS

Participants

Of the 243 infants seen at NICU Kolding during the daytime hours of the study period, 4 were excluded because of esophageal atresia, heart deformity, Pierre Robin syndrome, and meningomyelocele. In addition, the parents of 119 infants were not asked to participate in the study due to high workloads at the department, language difficulties, the infant not needing a heel stick, or the absence of a parent, and 21 families declined to participate in the study. Ultimately, 99 infants, representing 176 heel sticks, were eligible for randomization. Three additional infants were excluded after randomization but before completing the procedure. This left 173 heel sticks for inclusion in the study: 57 with glove warming completed in 50 infants, 59 heel sticks with gel pack warming completed in 47 infants, and 57 heel sticks with blanket warming completed in 46 infants. The outline and flow of this study are presented in Figure 1.

FIGURE 1.

Study flow. ^aOne heel stick was excluded because of lack of time for the COMFORTneo score and 1 was excluded because of sudden isolation due to Covid-19. ^bOne heel stick was disrupted because of very difficult blood sampling unrelated to the warming of the heel. n indicates number of heel sticks.

Baseline Data

The baseline demographic and clinical characteristics of the randomization groups are presented in Table 1. There were no statistically significant differences between the 3 groups, indicating that the randomization was successful. There were no significant differences in the work experience of BLSs between the 3 groups (P = .77); overall, 51 (29%) had less than 2 years, 50 (29%) had 2 to 5 years, 20 (12%) had 6 to 10 years, and 52 (30%) had 11 years of experience and more. In addition, there was no difference among the 3 groups in the subjective grading of the level of heel squeezing evaluated by the BLS (data not shown).

TABLE 1. Clinical Characteristics for the Infants in Whom the Heel Sticks Were Performed^a.

	Warming Items
	Glove	Gel Pack	Blanket
Heel stick, n	57	59	57
Age/median (minimum-maximum)
GA at birth (weeks + days)	36 + 6 (25 + 0-42 + 3)	34 + 5 (25 + 0-42 + 3)	36 + 5 (27 + 1-42 + 0)
Age at procedure, d	4 (2-69)	4 (1-64)	5 (1-7)
PMA at procedure (weeks + days)	38 + 1 (31 + 0-43 + 1)	36 + 2 (29 + 4-42 + 6)	38 + 1 (28 + 1-42 + 6)
Weight/median (minimum–maximum)
Latest, g	2940 (980-5076)	2600 (775-4616)	2775 (727-4882)
Sex, n (%)
Female	26 (46)	27 (46)	33 (58)
Location of infant, n (%)
Skin to skin	13 (23)	17 (29)	14 (25)
In the arms of a parent	11 (19)	4 (7)	1 (2)
Cradle	27 (47)	29 (49)	32 (56)
Incubator	6 (11)	9 (15)	10 (17)
Incision site, n (%)
Lateral	39 (68)	45 (76)	42 (74)
Medial	18 (32)	14 (24)	15 (26)

Abbreviations: GA, gestational age; PMA, postmenstrual age.

^aRaw numbers (or data?) without correction for clusters.

The primary and secondary outcomes of each warming method are presented in Table 2. There was no difference between the 3 warming methods in terms of blood flow velocity, hemolysis index, or COMFORTneo score. The appearance of the heel was documented for 154 procedures and was categorized as normal or skin injured (bruised or swollen skin). Most skin-injured heels were observed after using glove warming: 12 heels (23%) compared with 6 heels (12%) after gel pack warming and 4 heels (8%) after blanket warming, although the difference was not statistically significant (P = .07). The heel skin temperature after warming was included as a measure of the effect of the warming method. Figure 2a illustrates that the highest heel skin temperature was obtained when using glove warming and the lowest when using blanket warming. Skin temperatures varied significantly between the 3 warming methods (P < .001). The effect of each warming method is shown in Figure 2b. In the paired analysis adjusted for clusters, the change in heel skin temperature for each method was significantly different from zero (P < .0001).

TABLE 2. Comparison of Warming Methods.

	Warming Items
	Glove	Gel Pack	Blanket	P
Primary outcome
Blood flow velocity, μL/s
Unadjusted median (minimum-maximum)	4.4 (2.4-12.9)	4.5 (1.8-11.6)	5.0 (2.0-8.8)
Estimate (95% CI) adjusted for cluster	4.0 (3.6-4.6)	3.9 (3.5-4.4)	4.0 (3.5-4.5)	.91
Secondary outcomes
Hemolysis index, μmol/L
Unadjusted median (minimum–maximum)	27.5 (4-199)a	29.5 (2-138)a	27.5 (5-166)a
Estimate (95% CI) adjusted for cluster	27.3 (21.3-34.9)a	28.1 (21.8-36.2)a	27.5 (21.7-34.8)a	.99
COMFORTneo score during heel stick
Unadjusted median (minimum-maximum)	14 (10-26)a	14 (9-26)a	14 (9-22)
Estimate (95% CI) adjusted for cluster	14 (13-15)a	14 (13-15)a	13 (13-14)	.76
Postprocedure heel appearanceb				.07
Not examined	5	9	5
Normal, n (%)	40 (77)	44 (88)	48 (92)
Skin injured, n (%)	12 (23)	6 (12)	4 (8)

Abbreviation: CI, confidence interval.

^aOne missing data in group.

^bData only available for 154 procedures hence 19 were not examined.

FIGURE 2.

Heel skin temperature depends on warming method. Heel skin temperature after warming (panel a) and differences between heel skin temperatures from before warming to after warming (panel b). Individual values for each heel stick, raw medians, and boxes for interquartile ranges are presented. Statistical differences between either 2 or all 3 warming methods are ^aP < .01, ^bP < .0001, and n.s. indicates not significant.

Post hoc Analysis: Heel Appearance in Relation to Skin Temperature

As a post hoc exploratory analysis, we evaluated the relationship between heel skin temperature and postprocedure heel skin injuries, regardless of the warming method. Figure 3 illustrates that the increase in heel skin temperature was similar for both the normal (Figure 3a) and skin-injured (Figure 3b) heel appearance groups (P = .94). However, for heels with postprocedure skin injuries, the heel skin temperature after warming was significantly higher, as shown in Figure 3b (P = .038). The higher heel skin temperature could be due to a higher baseline heel skin temperature before warming (median [interquartile range] 37.80 [0.9]°C in skin-injured heels vs 37.10 [1.4]°C in normal heels), although this difference was not statistically significant (P = .21).

FIGURE 3.

Change in heel skin temperature in each heel appearance group. The dotted lines connect paired heel skin temperatures from before and after warming for each heel stick for normal heels (panel a, yellow dots) and skin-injured heels (panel b, red dots). Individual values for each heel stick, unadjusted medians, and boxes of interquartile ranges are presented for each heel appearance group.

The number of heel sticks resulting in postprocedure heel skin injuries increased with increasing heel skin temperature before warming, as illustrated in Figure 4. It is readily seen that most heel skin injuries, both absolute and relative, were observed when warming was performed at a baseline heel skin temperature above 37.0°C. The subjective grading of heel squeezing during the procedure, as registered by BLSs, was not associated with either the heel skin temperature after warming (P from regression = .10) or heel appearance 2 hours after the procedure (P = .99).

FIGURE 4.

Heel appearance in relation to heel skin temperature before warming. Yellow: number of heel sticks resulting in normal heel skin. Red: number of heel sticks resulting in skin-injured heel. Presented underneath are the total numbers of heel sticks displayed within sequential temperature ranges of 0.5°C and the percentage of heel sticks resulting in skin-injured heels in each.

DISCUSSION

In this study, we found a difference between the 3 warming methods in terms of the heel skin temperature obtained after completing the warming procedures. However, this did not translate into better procedure quality in any group. Despite differences in skin temperatures obtained using the 3 different methods, no differences between them in blood flow velocity, hemolysis index of the blood samples, COMFORTneo score, or postprocedure skin injuries were observed.

Because our study did not include a control group that did not receive warming prior to the procedure, we cannot infer that warming does not have a positive effect. The decision not to include a control group in the study was based on the fact that the warming procedure was already well implemented in the department and that both the BLSs and the neonatal nurses were convinced that the omission of this procedure would lead to inferior treatment of the infants. Because of this resistance, we agreed on a design without a control group to ensure feasibility of the study.

We chose blood flow velocity as a measure of procedure quality, considering that a high blood flow velocity would decrease the time the infant is exposed to the painful procedure, require less pressure on the incision site during the procedure, and provide a better sample quality less prone to hemolysis. Our study showed no difference between the 3 warming methods in terms of blood flow velocity. Unfortunately, it has not been possible to directly compare these findings with other studies, as they examined warming versus no warming conditions and did not compare the different methods of warming.

Moreover, we found no differences between the warming methods regarding the COMFORTneo score during heel sticks. However, we expected the infants' COMFORTneo scores to be higher than measured (14 or more indicates pain that requires an intervention21,27). The lower scores may be due to the care instructions already implemented at NICU Kolding, where infants are treated with oral sucrose or lactation, skin-to-skin contact, and/or are comforted by either parents or a neonatal nurse during the heel stick procedure. An observational study evaluating the effect of skin-to-skin contact in combination with other pain-relieving procedures during heel sticks found no significant difference in COMFORTneo when comparing the use of skin-to-skin contact with no skin-to-skin contact.31 This suggests that the infants already had pain relief from other procedures, such as oral sucrose.

In light of this, pain evaluation in this study could also be influenced by the pain-relieving procedures already offered to the infants, making it difficult to demonstrate any additional beneficial effects. Therefore, it can only be concluded that warming as a possible additional procedure did not influence the infants' COMFORTneo score. Other elements could also influence pain evaluation, as described by Schenk et al.32 They concluded that individual contextual factors such as behavioral state, ventilation status, and postmenstrual age have an impact on the infants' pain response and recommended that clinicians incorporate this in their evaluation to achieve a more accurate clinical pain assessment of the infants. Our study did not consider this because the protocol was approved prior to the publication of this study.

Regarding the correlation between warming of the heel and the level of squeezing applied, the literature addressing this topic is sparse, but in a randomized controlled trial, Janes et al33 found that more infants in the warmed group (P = .003) received squeezing during heel stick. This is surprising, given that warming is expected to reduce the need for pressure or squeezing due to increased capillary blood flow.17 In Danish clinical practice, there is a presumption that warming the heel is essential to reduce the heel squeezing during collection of capillary blood. Yet, our study did not identify any correlation between higher skin temperatures and the level of heel squeezing. Nevertheless, future studies are warranted on this topic.

We expected a nonwarmed heel to encounter more skin injuries, such as bruising or swelling, and to examine a potential relationship between heel skin temperature and postprocedure heel skin injury, a post hoc analysis was performed. Interestingly, we observed that the heel skin temperature after warming was significantly higher in infants with postprocedure heel skin injuries. This led to the investigation of the possible impact of the heel skin temperature before warming to elucidate whether this could be an indicator of whether the heel should be warmed. This showed that the number of heel sticks resulting in postprocedure heel skin injuries increased with increasing heel skin temperature before warming; this was especially true in cases in which the warming was performed on a heel with a baseline heel skin temperature above 37.0°C.

Thus, it appears that warming an already warm heel may cause skin injuries, resulting in discomfort for the infant. We hypothesized that since heat can induce capillary dilatation and increase blood flow,17 disrupting a dilated capillary during heel stick could easily promote bruises or swelling. A possible solution to this could be to measure the heel skin temperature before a heel stick to determine whether warming can be omitted. Gentle selective warming of only heels with low skin temperatures can be considered in the future. Overall, the warming of the infant's heel before the heel stick remains a debatable topic.

STRENGTHS AND LIMITATIONS

A major strength of the current study is the successful randomized and double-blinded design, both of which play an important role in reducing bias and improving the validity of the study results.34,35 In addition, the certification of the neonatal nurses in COMFORTneo scoring increased the comparability of the scores assessed by different nurses.21 The decision to have the PI ensure both computer-generated randomization and blinding procedures, as well as perform all warming procedures and closely supervise data collection, led to very little missing data, which also strengthened the study.

The point in time for visual assessment of heel injuries was set at 2 hours postprocedure to standardize data collection. We might have detected an increased number of heel skin injuries if the assessment time point had been set later. Data collection on postprocedure heel appearance was administered by the neonatal nurses and, due to high workload, was documented only for 154 procedures, which was a limitation of the study that lowered the statistical power for this outcome.

The study was designed to compare warming methods. The inclusion of a control group with no warming prior to the heel stick would have enhanced the evaluation of the warming method effect and allowed comparison of our results with the results from previous studies. Hence, a control group with no warming prior to the procedure would be valuable in future studies with similar designs.

CONCLUSION

Despite a significant difference in heel skin temperature after warming, with the glove providing the highest heel skin temperature, we found no difference between the methods in regard to blood flow velocity, hemolysis index, COMFORTneo score, or postprocedure heel skin injury. All 3 warming methods had similar effects with regard to the preanalytical quality of the blood sample as well as comfort for the infant. Irrespective of the warming method, we found that the number of heel sticks resulting in postprocedure skin injuries increased as the before warming skin temperature increased, with most injuries, such as bruising or swelling of the heel, observed when the baseline heel skin temperature was above 37.0°C. Since the CLSI guidelines still recommend the puncture site to be “warmed as needed,” our future procedure will be directed toward a gentle, standardized warming of only the heels with baseline skin temperatures measuring below 37.0°C. Further studies are needed to clarify the optimal heel skin temperature before the heel stick procedure and to identify other contextual factors that may influence the quality of blood samples and infant's comfort associated with heel sticks.

Summary of Recommendations for Practice and Research.

What we know:	There was no difference between the 3 warming methods in blood flow velocity, hemolysis index, COMFORTneo score, or postprocedure skin injuries. The number of heel sticks resulting in postprocedure skin injuries increased with increasing skin temperature.
What needs to be studied:	A control group not receiving warming should be included to evaluate the value of routinely warming of the heel prior to heel stick. Supplementary research toward the correlation between warming of the heel, level of squeezing the heel during procedure, and skin injuries to the heel. Clarification of the optimal skin temperature before the heel stick procedure. Uncovering if other contextual factors may influence the quality of the blood sample, and infant's comfort associated with heel sticks is advisable.
What can we do today:	Spare the warming if the infant's heel is already warm to avoid postprocedure skin injuries due to the heel stick. When warming is necessary, be aware to apply only moderate warmth to the heel.