Blood tests are the cornerstone of laboratory monitoring for patients in the intensive care unit but can result in significant blood loss. Modern laboratory analyzers require smaller volumes of blood than previous analyzers but most ICUs still use large volume blood tubes. A new study shows that ICUs can safely switch to smaller volume tubes that can reduce blood loss.
In the past, most collection tubes used for blood testing were made of glass with rubber stoppers and interior vacuum to facilitate blood filling. Currently, most hospitals primarily use plastic collection tubes that are safer than glass tubes. In the United States, the Becton Dickinson Corporation’s Vacutainer® is the dominate blood specimen tube used in most hospitals. Blood collection tubes are taken to the hospital’s clinical lab where tests are run on laboratory analyzers. The original analyzers required several milliliters of blood to perform testing, however the current generation of analyzers generally require only 0.5 ml of blood (or less) for testing. But blood specimen tube size has changed little over the past 40 years. As a consequence, most blood sent to the lab for testing is unused. Blood tubes come in a variety of sizes and each hospital determines the size of each type of tube to stock. The typical standard-sized Vacutainer® tubes used in most U.S. adult hospitals are as follows:
When drawn directly from a peripheral vein, no blood needs to be discarded for most laboratory tests. However, when drawn from a central venous catheter or a tunneled catheter, the initial aliquot of blood removed should be discarded since it can be diluted by electrolyte-containing IV fluids or catheter flushes used to maintain catheter patency. The recommended discard volume varies between hospitals but is typically 5 or 6 ml.
Let’s take a typical example of a patient with a central venous catheter admitted to the ICU with sepsis requiring regular blood draws consisting of a chemistry panel, CBC, and lactate every 6 hours plus a PTT/INR every 12 hours and a vancomycin level daily.
- Discard x 4 = 20 ml
- Chemistry x 4 = 28 ml
- Lactate x 4 = 16 ml
- Hematology x 4 = 12 ml
- Coagulation x 2 = 9 ml
- Drug level x 1 = 7 ml
- TOTAL DAILY VOLUME = 92 ml
The average 70 kg healthy adult has 5.3 liters of blood (5,300 ml). Therefore, blood loss for laboratory test specimens in this patient would be 1.7% of the blood volume per day. For a 5-day ICU stay, this would add up to 8.7% of the blood volume or 460 ml. This is also the volume of 1 donated unit of blood. Many critically ill patients are already anemic at the time of ICU admission and thus start off their ICU stay with a much lower amount of red blood cells than a normal healthy person, thus compounding the effect of blood removal for lab testing. This is a significant amount of blood and can affect oxygen carrying capacity of the blood to target tissues. It can also mean the difference between whether or not a patient needs a blood transfusion. The unfortunate irony is that most of this blood gets wasted.
In the past, the threshold hemoglobin level in the ICU to order a blood transfusion was 10.0 g/dL. More recently, that threshold has dropped to a hemoglobin level of 7.0 g/dL for most ICU patients based on clinical studies indicating that ICU patient outcomes are better when using the lower hemoglobin value. The effect of blood loss for laboratory testing will be greater for patients who are already anemic in whom lab testing-related blood loss is more likely to push the hemoglobin below the 7.0 g/dL level and thus require transfusion.
Reducing blood collection tube size
A study in this week’s JAMA examined whether ICU patient outcomes can be improved by changing to smaller Vacutainer® tube sizes. This was a large study involving 21,201 patients in 25 ICUs in Canada. Patients had specimens drawn into standard-size blood collection tubes (4 -6 ml) or small tubes (1.8 – 3.5 ml). Red blood cell transfusion was less common in patients assigned to the small volume tubes (10 units of RBCs fewer per 100 patients). The drop in hemoglobin level was also lower in small volume tube patients. The frequency of specimens with insufficient quantity of blood for analysis was not higher using low volume tubes (in fact, it was statistically significantly lower when using the smaller-size tubes).
The implication of this study is that ICUs can safely shift to use of small volume blood collection tubes without risk of an insufficient quantity of blood to perform laboratory tests. By changing to small volume tubes, ICU patients have a smaller ICU-related drop in hemoglobin and require fewer units of transfused blood during their ICU stay. This should come as no surprise to pediatricians who have successfully used smaller volume blood tubes for many years to perform the exact same blood tests ordered in adult ICUs.
Reducing blood discard volume
A 2021 study in the Journal of Laboratory Physicians examined whether a 3 ml blood discard volume was as good as a 5 ml blood discard volume in patients with central venous catheters. There was no significant difference in chemistry lab test results between specimens obtained after the two discard volumes. The implication is that ICUs can shift to using smaller discard volumes, thus further reducing ICU-related blood loss.
A typical triple lumen central venous catheter has 3 lumens that are each either 18 gauge (1 mm) or 16 gauge 1.3 mm) in diameter. The catheter length is selected based on which central vein is chosen for insertion and is either 15 cm or 20 cm. The volume of each lumen can range from 0.12 ml to 0.27 ml, depending on lumen gauge and catheter length. PICC line lumens are typically 18 gauge and are 50 – 60 cm in length. This equate to a lumen volume of 0.47 ml. Thus, even using a 3 ml blood discard volume allows for several times the volume of fluid contained in the catheter to be removed before obtaining venous blood through a triple lumen central venous catheter or a PICC line.
Implications for ICU policies
When it comes to blood draws for lab testing in the intensive care unit, less is more. We can reduce ICU-related blood loss and reduce blood transfusions in ICU patients with several policy changes.
- Reduce standard central venous catheter blood discard volume to 3 ml.
- Change to small volume Vacutainer® tube sizes. The vast majority of laboratory tests can be performed using 1.8 to 3.5 ml tubes.
- Consolidate blood draw time intervals. Avoid ordering some blood tests at 6 hour intervals and others at 8 hour intervals – this results in six or seven blood draws per day, wasting excessive blood for discards and occupying valuable nursing time.
- De-escalate lab testing as soon as clinically indicated. The need for lab tests and test frequency should be reassessed on a daily basis and incorporated into multidisciplinary rounds checklists.
Simply reducing the blood discard volume to 3 ml and changing to smaller volume blood collection tubes should reduce ICU-related blood loss from lab testing by 50%. These policy changes should not be made unilaterally by any one hospital leader. Ideally, these should arise from a consensus of the laboratory medical director, the ICU medical director, the ICU nurse manager, and the director of hospital supply. The reasons for the change should also be communicated to all of the nurses and physicians who practice in the ICUs in order to ensure a consistent message to the hospital staff. Smaller volume blood collection tubes can also be well received by patients and families who in the past have frequently raised concern about the volume of blood regularly removed from ICU patients for lab test purposes.
There is always resistance to change in our intensive care units. We get comfortable with certain practices because that is the way that we have always done things and because we fear that any change could have deleterious effects on patient outcomes. Change in the ICU should be steered and not forced. New findings in the medical literature now make reducing blood loss for lab testing a change that will be much easier to steer.
November 24, 2023