Key messages
- We found limited evidence of poor quality for using blood collected from a vein to detect altered levels of oxygen, carbon dioxide, and acidity in patients with respiratory failure.
- Compared to blood taken from an artery, taking blood from a vein would result in more patients testing positive for abnormalities when they do not have them (false positives), which would result in most patients also needing blood collected from an artery.
Why is it important to improve the collection of blood to diagnose abnormalities in oxygen, carbon dioxide, and the acidity of the blood in people with respiratory failure?
When a person is unwell, there is a change in the levels of oxygen, carbon dioxide, and other substances in the blood. This can be detected by analysing blood collected from an artery ('arterial'). By measuring the precise amount of oxygen and other substances in arterial blood, medical professionals can diagnose (and treat) life-threatening conditions such as respiratory failure, high blood levels of carbon dioxide, and altered acidity of the blood (also called metabolic disturbance).
For reasons of convenience and better patient comfort, some medical professionals utilise venous blood (collected from a vein of the arm or leg) instead of arterial blood.
What is peripheral venous blood gas analysis?
Peripheral venous blood gas analysis is the analysis of blood collected from a peripheral vein of the arm or leg and measured in the usual way with a blood gas analyser.
If this test can detect all cases of people with altered levels of blood oxygen, carbon dioxide, and acidity in the blood, the number of patients requiring the puncture of an artery, which is more painful and difficult to carry out, would be reduced. If venous blood testing were also precise in identifying patients without these blood alterations, the puncture of an artery could be avoided.
What did we want to find out?
We wanted to find out the accuracy of gas analysis on blood taken from a vein instead of an artery in people with suspected respiratory failure or change in blood acidity due to underlying illness or abnormality.
What did we do?
We searched for studies that included people with suspected respiratory failure, high blood levels of carbon dioxide, and altered acidity of the blood, who were admitted to hospitals or emergency departments. We combined results across these studies.
What did we find?
We included six studies with 919 adults (895 with reported results) who were suspected of having respiratory failure, high blood levels of carbon dioxide, and altered acidity of the blood, from a variety of settings including hospitals and emergency departments.
In 1000 patients, of whom 330 have respiratory failure (confirmed by arterial blood testing), the diagnostic accuracy of VBGA would be as follows:
- 325 would test positive and would have respiratory failure treated appropriately;
- 431 would test positive but not actually have respiratory failure (false positives), and the consequences would be inappropriate or harmful treatment;
- 244 would test negative, of which 239 would be true negatives and would avoid arterial puncture; and
- 5 would actually have respiratory failure (false negatives), and the consequences would be missed diagnosis and treatment.
Similarly, in 1000 patients, of whom 330 have high blood levels of carbon dioxide:
- 324 would test positive and would have high blood levels of carbon dioxide treated appropriately;
- 311 would test positive but not actually have high blood levels of carbon dioxide (false positives), and the consequences would be inappropriate or harmful treatment;
- 365 would test negative, of whom 359 would be true negatives and would avoid arterial puncture; and
- 6 would actually have high blood levels of carbon dioxide (false negatives), and the consequences would be missed diagnosis and treatment.
What are the limitations of the evidence?
The test accuracy results presented in this review come from only six studies and are not precise, particularly in identifying patients with no blood gas and metabolic disturbance. Moreover, the selection of patients and test process were not reported in detail, and we are concerned about the reliability of the study findings.
How up to date is this evidence?
The evidence is based on searches run to July 2024.
Read the full abstract
Arterial blood gas analysis (ABGA) is the reference standard for the diagnosis of respiratory failure (RF) and metabolic disturbance (MD), but peripheral venous blood gas analysis (PVBGA) is increasingly being used for the estimation of carbon dioxide, pH, and other variables in the context of acutely unwell adults presenting to hospitals and emergency departments.
Objectives
The primary objective of this review is to evaluate the performance of PVBGA by comparing it with the reference standard ABGA, which is assumed to be error-free for the diagnosis of (1) respiratory failure, (2) hypercarbia, and (3) metabolic disturbance (the three target conditions) in adults.
The secondary objective is to evaluate the performance of the index test to diagnose nine specific subtypes of respiratory failure and metabolic disturbance. The definitions for these additional conditions are determined by changes to one or more of the following: pH (acidity), pO2 (partial pressure of oxygen), pCO2 (partial pressure of carbon dioxide), HCO3 (bicarbonate), as stated in the Methods section of this review (target conditions). We aimed to explore the following covariates: participant demographics (e.g. age, weight, and sex); participant comorbidities (e.g. chronic lung disease, chest wall deformity, and central nervous system disorder such as spinal cord injury); and the indication for blood gas sampling (e.g. shortness of breath, critical illness, resuscitation, trauma, or whilst under general anaesthesia).
Search strategy
On 10 July 2024, we searched the electronic databases MEDLINE, EMBASE, CINAHL, and LILACS. We also manually searched 19 respiratory and critical care journals, and we searched ClinicalTrials.gov for ongoing trials.
Selection criteria
We considered consecutive series studies and case-control studies that directly compared the index test PVBGA to the reference standard ABGA for adults over the age of 16 years. The included studies contained data for any one of the target conditions of respiratory failure and metabolic disturbance, as determined by individual changes to pO2 (partial pressure of oxygen), pCO2 (partial pressure of carbon dioxide), pH (acidity), and HCO3 (bicarbonate) concentration. Studies that only provided mean values for summed data were ineligible for inclusion. However, we invited authors of such studies to provide individual patient data for inclusion in this systematic review. There are nine studies awaiting classification.
Data collection and analysis
Two authors independently evaluated the quality of the relevant studies and extracted data from them. We conducted a quality assessment using the QUADAS-2 tool. Our statistical analysis used 2 x 2 tables for the positive and negative results of each test. We estimated a bivariate meta-analysis of sensitivity and specificity.
Main results
We included six studies (919 participants) in our quantitative analysis. All studies were at high risk of bias due to one or more of the following factors: patient selection, since it was unclear if consecutive patients were included or where they were located; index test, with poor reporting of cut-offs; flow and timing domain because the fraction of inspired oxygen was frequently not stated and any difference between the collection of the VBGA and the ABGA could introduce bias.
Respiratory failure
For the diagnosis of respiratory failure of any type, when using PVBGA, the estimated summary sensitivity (Sn) was 97.6% (95% credible interval (CI) 94.1 to 99.4) and the estimated summary specificity (Sp) was 36.9% (95% CI 17.1 to 60.1) (6 studies, 805 participants of whom 291 (36%) were diagnosed with respiratory failure by ABGA; sensitivity: low-certainty evidence; specificity: very low certainty evidence).
Isolated hypercarbia
For the diagnosis of isolated hypercarbia (regardless of oxygen level), when using PVBGA, the estimated summary Sn was 97.1% (95% CI 93.3 to 99.2); the estimated summary Sp value was 53.9% (95% CI 39.8 to 66.7) (6 studies with 805 participants, 269 (33%) with ABGA confirmation; low-certainty evidence).
Other findings
Results for metabolic disturbance and our secondary target conditions are presented in the full review.
Authors' conclusions
Very limited data suggest PVBGA performs poorly as a diagnostic test for respiratory failure compared to the reference standard of ABGA. The index test PVBGA was highly sensitive for the diagnosis of respiratory failure and isolated hypercarbia, but its specificity was poor for these two primary target conditions. The high sensitivity means PVBGA may have a useful role as a "rule out test" for respiratory failure and isolated hypercarbia; however, the high false-positive rates make the clinical interpretation of a positive test difficult. Moreover, we are uncertain regarding these estimates because we have only low to very low certainty about the evidence. Further studies that use (ABGA) established thresholds for the diagnosis of each target condition are needed.
