Drawing blood is a common medical procedure that is performed routinely in healthcare settings. When a needle is inserted into a vein to collect a blood sample, the first drop of blood that emerges is usually wiped away before collecting the sample into a vial. This is standard practice, but many patients wonder why that initial drop of blood is discarded.
What is the purpose of wiping away the first blood drop?
There are a few main reasons why the first drop of blood is wiped away when drawing blood:
- To avoid contamination from alcohol or other antiseptics used to cleanse the skin
- To avoid diluting the blood sample with tissue fluid or interstitial fluid
- To obtain a pure sample of venous blood that is not altered by the needle stick
Let’s explore each of these reasons in more detail:
Avoiding contamination from skin disinfectants
Prior to inserting the needle, the skin is cleaned with an antiseptic solution like alcohol or iodine. This is done to disinfect the skin and prevent infection. However, trace amounts of the antiseptic can be carried into the needle along with the first drop of blood. Wiping away the first drop prevents contamination of the sample with any residual skin disinfectant.
Avoiding dilution with tissue fluid
When the needle first punctures the vein, it will collect some amount of interstitial or tissue fluid before redirecting the flow of blood. Tissue fluid exists outside of blood vessels and has a different composition than venous blood. If the first drop of blood is not wiped away, the sample will be diluted with tissue fluid. This could alter test results. Discarding the first drop ensures a pure sample of blood is collected without any diluting tissue fluid.
Obtaining pure venous blood
The first drop that emerges from the needle stick may also contain damaged cells and platelets from where the needle pierced the vein. It may mix with a small amount of arterial blood as well. Wiping this initial drop away reduces the collection of damaged blood components and helps obtain a pristine sample of venous blood for analysis or testing.
Does the first drop need to be wiped if using a butterfly needle?
Butterfly needles, also called winged infusion sets, are commonly used for drawing blood. The needle is very short and connected to long flexible tubing. Some people wonder if the first drop wipe is still needed with a butterfly collection system.
The answer is yes – the initial drop of blood should still be wiped away even if using a butterfly needle. Here’s why:
- There is still potential for contamination from skin disinfectants
- Dilution with tissue fluid can still occur
- Damaged blood components from the needle stick will still be present
The only exception would be if a prime tube is first filled to remove the diluted blood before collecting samples. If this prime tube is used, the first wipe may not be needed. But most procedures will still include wiping the first drop as standard protocol.
Does the first drop need to be wiped for all blood tests?
Wiping away the initial blood drop is standard for nearly all blood testing scenarios. However, there are some exceptions where the first drop does not necessarily need to be discarded:
- Blood cultures to check for infection – here, contaminants may actually be helpful to assess for pathogens.
- When doing a bleeding time test – this specifically measures how fast blood flows from a puncture wound.
- If using a prime tube to remove the first several drops before collecting samples.
In these situations, the first undiluted blood drop may be required for accurate test results. But the majority of the time, it is good practice to wipe away that first drop of blood before gathering samples for testing.
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What techniques should be used when wiping away the first blood drop?
Proper technique is important when wiping away the initial blood drop to avoid compromising the sample:
- Use sterile gauze or an applicator to absorb the blood – do not touch the puncture site.
- Apply gentle pressure – do not squeeze or milk the vein.
- Allow a new drop of blood to form before collecting samples.
- Avoid re-palpating or re-probing the vein after the first wipe.
It is also crucial to properly dispose of the used gauze in a biohazard container. Proper hand hygiene should be performed and gloves changed before moving forward with sample collection.
Why avoid milking or squeezing the vein?
It is important not to forcibly squeeze or milk the vein when wiping away the first blood drop. This can damage the vein and contaminate the sample with tissue fluid. Gentle blotting or absorption of the initial drop is all that is needed. Forcing or squeezing the blood flow can jeopardize the accuracy of results.
Does the first drop need to be wiped for fingerstick blood samples?
Fingerstick blood sampling uses a lancet device to obtain a small drop of capillary blood from a fingertip. This method is frequently used for point-of-care testing of blood glucose, hemoglobin, INR, and other analytes.
In most cases, the initial blood drop should also be wiped away before collecting a fingerstick sample. This helps avoid excess tissue fluid and ensure an undiluted sample of capillary blood.
However, some fingerstick devices are designed to automatically discard and reload the first drop. In this case, manually wiping is not necessary. It is best to follow the manufacturer instructions for proper technique with these collection devices.
Exceptions for first drop wipe with fingerstick sampling
There are a few exceptions where the first blood drop can be used for fingerstick sampling:
- If using a lancet and test strip without a discard feature – the first drop is directly applied
- When blood flow is limited due to low blood pressure, dehydration, or cold temperatures
In these situations, the initial fingerstick drop is often needed to ensure sufficient sample volume. But when possible, it is still preferred to discard the first drop whenever fingerstick sampling.
Does wiping the first drop affect test results and accuracy?
Wiping away the initial blood drop can improve test accuracy and prevent erroneous results in many cases. Here are some of the potential effects if the first drop is not discarded:
- Hemolysis from damaged blood cells, causing inaccurate counts
- Dilution from tissue fluid, altering analyte levels
- Contamination from antiseptic agents
- Platelet clumping or activation from the needle trauma
All of these factors can skew test results if that first drop of blood is not wiped. For example, potassium or iron levels may read falsely low or high. Complete blood counts may be impacted by hemolysis. And antiseptics can interfere with certain chemical analyses.
When can test results remain accurate without wiping first drop?
There are select cases where test results may not be significantly impacted if the first blood drop is not wiped. This includes:
- Blood typing – the ABO/Rh result will still be accurate.
- Toxicology screening – drugs or toxins will still be detectable.
- Blood cultures to detect bacteria or viruses.
However, for most blood tests, wiping that initial blood drop will improve the clarity and precision of the final results.
Does the first wipe increase the risk of infection or complications?
There is minimal increased risk of infection or complications from discarding the first blood drop. Proper precautions include:
- Using a sterile applicator to absorb the initial blood drop.
- Applying gentle pressure – no forceful milking of the vein.
- Disinfecting again before making a second needlestick (if required).
When these guidelines are followed, there is little added risk introduced by wiping away the first drop. The first wipe is generally done promptly in a smooth motion without removing the needle fully.
Any extra needle manipulations, probing, or palpation of the vein after the first wipe could potentially increase risks like bruising or infiltration. But a simple wipe itself does not significantly increase complications when done appropriately.
Special precautions in high-risk patients
Extra care should be taken when wiping the first blood drop in patients at higher risk for bleeding or infection:
- Immunocompromised patients
- Those on anticoagulants with higher risk of hematoma
- Children and elderly with delicate veins
In these cases, use the minimum pressure necessary and avoid any forceful squeezing when absorbing the initial blood drop. Be prepared to apply pressure or bandage the site if any bleeding occurs after wiping.
Wiping away the first drop of blood during sample collection is a standard, recommended practice. Discarding the initial drop helps avoid contamination, dilution, and damaged blood components that could impact test results. However, there are a few exceptions based on the type of test.
Proper technique is important when wiping away the first blood drop to prevent compromising the sample. With appropriate precautions, this standard practice poses little added risk for most patients while improving the quality of the blood sample.