Drawing Blood From Saline Lock

Introduction

Venepuncture carries a certain amount of pain and a small risk of complications. Given many patients have an in-situ peripheral intra-venous cannula, sampling blood from this obviates the need for repeated venepuncture if there are clinically equivalent and reliable results. In general, this is not common practice due to concerns regarding the validity of results.¹ Other considerations include the method used for obtaining samples, prior or concurrent use of the cannula for fluid administration, the aspiration volume needing to be discarded and any need for special equipment.

Whether assay results from peripheral intravenous cannula and venepuncture are clinically equivalent is quantified using limits of agreement, expressed as the range within which 95% of values will lie in comparison to a reference standard measurement.² The limits of agreement are then compared with a clinically acceptable range usually defined by consensus.

In order to assess the quantity and quality of studies comparing blood samples from existing PIV with venepuncture, we undertook a systematic review. We focused on studies of sufficient quality to influence clinical practice.

Methods

Search strategy

Relevant keywords and terms were developed through a scoping search in PubMed, eventually expanding to three databases (PubMed, ISI Web of Science and Embase). Two reviewers screened titles and abstracts (FL and DL). The abstracts were categorised into 'not relevant' and 'potentially relevant' and all 'potentially relevant' studies were reviewed in full. References of included studies were also hand searched (Figure 1 ). The full search strategy for PubMed is shown in Appendix 1.

Figure 1. Flow diagram showing search strategy. Source: Adapted from PRISMA flow diagram.¹¹

Study selection

Inclusion criteria

1. Studies that compared human blood samples drawn from peripheral intravenous cannulae and venepuncture.

2. Studies reporting numerical results for at least one of the following tests: sodium, potassium, chloride, urea (or blood urea nitrogen), creatinine, haemoglobin, haematocrit, white cell count, platelets, international normalised ratio, pH, partial pressure oxygen, partial pressure carbon dioxide.

3. Studies using the Bland–Altman method for limits of agreement or providing data which allowed the calculation of limits of agreement.²

Exclusion criteria

1. Articles not in English.

2. Studies which used newly inserted peripheral intravenous cannula for blood sampling unless intravenous fluids had been infused through the cannula prior to sampling.

3. Studies which took samples while infusions were running through the cannula or did not wait after stopping infusions.

4. Delay of greater than 5 min between samples for comparison.

5. Studies which did not discard at least 2 mL of aspirate prior to blood sampling.

6. Studies which required special equipment for blood sampling from peripheral intravenous cannula (for example double stopcock techniques).

Data collection and extraction

Relevant data were extracted from included papers in duplicate (FL and DL), including publication year, patient population, number of patients, blood tests carried out, discard volume prior to sampling, wait time between stopping infusions and sampling, aspiration method and cannula gauge as well as the assay results. Tests measured in non-SI units were converted to SI units. Blood urea nitrogen results were converted to urea by multiplying by 2.14 and then converting to SI units.

The 95% limits of agreement was the primary outcome of interest. If not reported directly, it was calculated

$$\begin{array}{c}\text{Limits}\text{of}\text{agreement}=\text{Meandifference}\pm 1.96\\ \times \text{SDofdifferencesfrommean}\hfill \end{array}$$

Limits of agreement values were then pooled using inverse variance weighting

$$\begin{array}{c}{\text{variance}}_{(\text{pooled})}\\ =\frac{{(\text{variance}}_{\text{study}1}\times {\text{n}}_{\text{study}1})+{(\text{variance}}_{\text{study}2}\times {\text{n}}_{\text{study}2})\dots }{{(\text{n}}_{\text{study}1}-1)+{(\text{n}}_{\text{study}2}-1)\dots }\hfill \end{array}$$

Clinically acceptable limits of agreement

The clinically acceptable errors in blood sampling are not fully established and vary depending on patient situation and the clinicians' tolerance for error. Four studies specified such ranges established through clinician survey.^3–6 We used a mean of these values to define clinically acceptable limits for this review.

Results

Literature search provided 1857 articles for abstract review with 130 duplicates (Figure 1 ). Hand-searching identified a further six studies for abstract review. There were 21 papers which were excluded at full text review for failure to meet the inclusion criteria or meeting the exclusion criteria (Appendix 2). Ultimately, seven studies were included with total individuals n = 746 from a combination of adult inpatient, adult emergency department, paediatric inpatient and healthy volunteers (Table 1).

Table 1. Summary of included studies.

Characteristics of included studies

Studies comparing venepuncture to peripheral intravenous cannula used different methods and protocols (Table 1). The minimum discard volume in the studies was 2 mL. Cannula sizes varied from 16 to 22 French. All cannulae had been used prior to sampling but there was variation in volume and contents of infusion prior to sampling. Sampling devices used were either syringe or vacutainer systems.

Biochemistry

Sodium, chloride, urea and creatinine pooled limits of agreement were all within the clinically acceptable error range (Table 2). In some cases, the pooled limits of agreement was substantially lower than the clinically acceptable error range (e.g. creatinine in µmol/L limits of agreement = −13, +12; clinically acceptable range = ±26). However, potassium limits of agreement exceeds the clinically acceptable error range (in mmol/L limits of agreement = −0.48, +0.46; clinically acceptable range = ±0.35).

Table 2. Results for renal function and electrolytes. Showing 95% limits of agreement for included papers, pooled 95% limits of agreement and clinically accepted range for comparison.

Haematology and international normalised ratio

The pooled results for haematology and international normalised ratio were all within the clinically acceptable limits of agreement (Table 3).

Table 3. Results for haematology and international normalised ratio (INR). Showing 95% limits of agreement for included papers, pooled 95% limits of agreement and clinically accepted range for comparison.

Table 4. Results for blood gas and pH. Showing 95% limits of agreement for included papers, pooled 95% limits of agreement and clinically accepted range for comparison.

Blood gases and pH

The pooled limits of agreement for pH was within clinically acceptable range (Table 4). However, the pooled limits of agreement for pCO₂ exceeded the clinically acceptable range (in kPa limits of agreement = −0.9, +1.3; clinically acceptable range = ±0.7) and limits of agreement pO₂ dramatically exceeded the clinical acceptable range (in kPa limits of agreement = −4.5, +3.0; clinically acceptable range = ±0.7).

Discussion

Across pooled studies, we showed that assays from used peripheral intravenous cannula were reliable and clinically consistent with fresh venepuncture samples, except in the case of potassium and blood gases. Taken together, our findings suggest that peripheral intravenous cannula sampling could be given greater consideration in clinical practice – at least for the tests described.

The results for potassium levels were not within clinically acceptable agreement limits for some patients. The 95% limits of agreement for potassium of −0.46 to + 0.47 mmol/L shows that, for patients where a tight control of potassium is essential, samples from used peripheral intravenous cannula should be used with caution. For most other patients, a sample from a peripheral intravenous cannula would be sufficient and the level of error ±0.47 mmol/L is unlikely to affect patient outcomes. The cause for the higher level of error is not clear, though haemolysis or haemodilution was excluded as causes in the studies considered here. For blood gas analyses (pO₂ and pCO₂), the two studies reporting these suggested errors were due to contamination of the samples with atmospheric air post-collection.^6,7

There were some limitations to this study. There was heterogeneity in study populations, protocols and equipment. It is not clear whether our findings are generalisable to other sampling techniques, e.g. with narrower gauge cannula.

In terms of haemolysis degrading the sample quality, we show that for most blood tests it does not lead to significant errors. However, if the laboratory or analysers do not check for haemolysed samples it could lead to errors in results.⁸

We did not assess some commonly ordered blood tests. Our findings relate to specific assays and may not be generalisable to other haematology and biochemistry investigations.

The clinical impact of these findings will be greatest in those situations in which patients require repeated blood tests where samples from peripheral intravenous cannula would be suitable. For example, if a patient were admitted with symptomatic anaemia and needed serial haemoglobin measurements samples from a cannula could be used. Peripheral intravenous cannula sampling can be an alternative for patients who find venepuncture intensely distressing. There are also patients in which venepuncture is technically difficult and peripheral intravenous cannula samples can provide easier access to blood.

Our findings do not explain why some blood tests are not reliable when taken from a used peripheral intravenous cannula and this could be the subject of further research. Further studies could also be considered to assess other assays which were not included in this paper.

Conclusions

Peripheral intravenous cannula samples are interchangeable with venepuncture for sodium, chloride, urea, creatinine and haematology tests. Peripheral intravenous cannula samples can be used for potassium measurement in situations where error of ±0.47 mmol/L is acceptable. Blood gas analysis for pO₂ and pCO₂ can show clinically significant differences between peripheral intravenous cannula and venepuncture and so peripheral intravenous cannula samples should not be used. Overall, peripheral intravenous cannula sampling is a reasonable clinical practice for a range of common assays.

Declarations

Competing Interests

None declared.

Funding

DD is supported by a Wellcome Trust Intermediate Clinical Fellowship (WT107467). DL is supported as a UCLH CEO clinical research fellow.

Ethics approval

Not applicable.

Contributorship

FL and DL both conceived the research. FL led the planning of the research. FL and DL contributed equally to the literature search, review of literature and data analysis. All authors were involved in drafting and have approved the final version. FL is the guarantor and principal investigator, accepting responsibility for the study.

Provenance

Not commissioned. Peer-reviewed by Devaki Nair and Patrick Twomey.

References

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Appendix 1. PubMed search strategy

1. ((cannula) OR saline lock device) OR peripheral venous catheter OR (Catheterization, Peripheral*) OR Infusions, Intravenous/instrumentation*

2. ((blood sampling) OR Blood collection) OR phlebotomy

3. Humans[MeSH Terms]

4. #1 AND #2 AND #3

Appendix 2. Excluded papers at full text review and reasons for exclusion

References

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Drawing Blood From Saline Lock

Source: https://journals.sagepub.com/doi/abs/10.1177/2054270419894817

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