Posted 30 January 2019
Posted 30 January 2019
Expert advice
How can I make sure my blood samples aren't rejected?
Underfilled tubes of blood could result in inaccuracies that affect patient care
Underfilled tubes of blood could result in inaccuracies that affect patient care
When we curated the @NHS Twitter account for a week last summer we sought to help raise the profile of healthcare scientists and give health professionals, patients and the public more insight into what happens in a hospital pathology laboratory.
Surprisingly, the most popular tweet we put out was a question about a blood sample we had rejected.
We asked: ‘Would you be disappointed if this blood sample was rejected?’ and included a photograph of an anonymised underfilled (to approximately 65%) ‘blue top’ sodium citrate sample tube, used to collect blood for coagulation studies.
Fill to the line
The response we got from clinicians was huge. While the majority seemed to agree that the sample was underfilled and would therefore be rejected, not everyone understood why, or why it is so important that these blood samples are filled to the line.
‘Selecting the wrong colour top, and therefore the wrong additive, could result in clinically significant inaccuracies in the results’
Blood tubes have different coloured lids for good reasons – the different colours represent the different additives inside the tubes, and are denoted by an international standard which is designed to preserve and protect the integrity of blood samples before, during and after analysis.
It is important that the appropriate additive is selected when collecting a blood sample for a particular pathology investigation. So selecting the wrong colour top, and therefore the wrong additive, could result in clinically significant inaccuracies in the results, with potentially serious consequences for the patient.
Investigating a clotting profile
The ‘blue top’ tubes contain a measured amount of buffered trisodium citrate solution, often referred to in the healthcare setting as ‘sodium citrate’, just ‘citrate’ or simply a ‘blue top tube’.
The natural process of blood clotting requires calcium ions. The sodium citrate in this tube, when mixed with freshly collected blood, prevents this process by withholding the calcium ions in the blood sample.
When we receive the blood sample in the laboratory, part of our procedure for investigating a clotting profile is to reintroduce calcium ions from a synthetic source – usually a standard reagent.
Mixing the right amount is critical
Common coagulation tests such as prothrombin time (PT) and activated partial thromboplastin time (APTT) are performed by adding a measured amount of calcium to the plasma of the anticoagulated blood sample.
This is expected to replenish the withheld calcium ions so that the laboratory-induced clotting can be initiated and measured. As this is a measure-for-measure test, it is critical to mix the right amount of blood with the right amount of citrate.
Tube manufacturers provide a fixed and validated volume of sodium citrate which is to be mixed with the patient’s blood, at a fixed ratio of 1:9 (citrate solution to whole blood).
The marker line you see on the tubes is a guide to the volume of blood required to achieve this ratio. In BD tubes – the ones we use – this is commonly a frosted line in the plastic, which is the minimum fill line.
Collecting a volume of blood that is less than what is required can alter the citrate solution to whole blood ratio. The leftover citrate, which is still active, has the potential to interfere with the calcium ions that will be added on testing.
This causes uncertainty about the validity of the results we can produce in the lab – it could lead to falsely prolonged clotting times, for example.
Need to be stringent
Similarly, collecting blood from a wing set or intravenous extension can draw air into the tube, resulting in inadvertent underfilling and an insufficient volume of blood being collected.
Overfilling these tubes can also cause alterations to the ratio of citrate and blood. In tubes with a vacuum, however, you can only significantly overfill them by taking the cap off, which is against the manufacturer’s advice and should never be done.
Is there room to be less stringent? Not really. Our advice to anybody collecting blood into a ‘blue top’ tube at our trust is to always collect a sufficient volume to at least meet or exceed the frosted line.
‘You may have heard that 10% below the guide line might be acceptable’
This advice may vary, depending on the manufacturer your trust acquires tubes from, so check with your pathology department if you are unsure.
Although you may have heard that 10% below the guide line might be acceptable, studies have shown that clinically significant changes to results occur at different levels of underfill for different tests.
With APTT, for example, the guide line represents 90% of the required volume. If you deduct 10%, that leaves 80% of the required volume in the blood tube, yet research shows that clinically significant changes to the result of the APTT can occur when less than 89% of the required volume is collected.
This is why we sometimes have to reject specimens.
What if it isn’t possible to draw enough blood?
During our week on the @NHS Twitter account many clinicians shared stories of frustration around ‘difficult to bleed’ patients. Difficult peripheral venous access can make it hard to aspirate a sufficient volume of blood. What if it simply isn’t possible to draw enough blood?
- RELATED: Safe transfusion of blood components
You could consider selecting a different size of ‘blue top tube’. For a ‘standard’ sodium citrate tube, 2.7ml of whole blood is required, but there may be other sizes available depending on the manufacturer your trust acquires blood tubes from.
At our trust we can provide clinical colleagues with tubes that require as little as 1.8ml of blood while still achieving the required ratio. Conversely, for investigations that might require a greater plasma yield, we are able to provide a larger tube that can collect 4.5ml of blood. The pathology department at your hospital can let you know what is available where you work.
No specimen is rejected lightly
Like nurses, biomedical scientists are heavily regulated and have standards of proficiencies we must adhere to. Although we rarely see patients, we take great pride in providing the best care we can, and always strive to produce accurate results.
Unsatisfactory results provide little value in the diagnosis or monitoring of patients. If no action or unnecessary action is taken on the back of an inaccurate result, it could cause harm.
No specimen is ever rejected lightly – it is always done in the best interests of the patient.
Daniel Gaskin and Ola Yahaya are biomedical scientists in the haematology and transfusion laboratory at Furness General Hospital, University Hospitals of Morecambe Bay NHS Foundation Trust
Acknowledgment: Adam Stretton, clinical manager UK & Ireland, BD Life Sciences – Preanalytical Systems
Further information
- @NHS website
- @NHS Twitter page
- Study on underfilled specimen tubes (ResearchGate)