Purpose: To discuss needle and syringe dead space and how it can lead to compounding errors to the final compounded product.
Discussion:
Every syringe is calibrated for accuracy to the markings on the syringe. Even the tip of the syringe where residual drug remains after use is included in the calibration of the syringe. As an example, if 0.5 mL of drug is pulled into a syringe and then injected into an IV bag, there is an assurance that 0.5 mL has been delivered into the bag (even though there is a small residual amount in the syringe tip).
However, compounding inaccuracies occur when needles are changed/added during compounding (or during administration). When a “dry” needle is added to a syringe, the compounder must take the needle’s dead space into consideration during compounding. In most situations, this needle dead space is clinically insignificant, the exception being small doses under 1 mL or drugs with a narrow therapeutic range. To avoid compounding errors with clinical significance that can negatively impact the patient, consistency must be introduced for all medications when needles are changed or added during compounding.
Not every needle will have the same dead space volume. Needles used for administration are larger gauge needles, that are smaller in diameter, and usually have a needle dead space of about 0.05 mL. On the contrary, compounding needles are smaller gauge needles, which are larger in diameter, and will have a larger needle dead space, usually about 0.1 mL. These dead space estimates are speculation, and the true dead space volume also depends on the length of the needle. The best avenue to determining the exact dead space of a needle is by inquiry to the manufacturer and/or measuring the volume of each needle size/manufacturer at the facility.
If a needle is attached to a syringe and used to enter a drug vial and then with the same needle is used (this is considered a "wet needle") to inject into a bag, then the amount of drug delivered to the final preparation will be what the compounder pulls back to the calibrated measured markings. Otherwise stated, when a syringe-needle combination is used together to draw and instill drug there is no concern for needle dead space inaccuracy. The concern arises only when the needle is added (as when using a dispensing pin) or changed (as when using a filter needle/straw, etc.).
When adding/changing a needle during compounding, some may argue that the best process for ensuring that all medication is delivered to the final preparation is by “flushing” the needle by aspirating the plunger. The process of "flushing" is not 100% accurate since the syringe hub volume is also introduced, which may or may not have a clinical impact. Additionally, if the process of “flushing” the needle is not done correctly or is misunderstood by the compounder, then it could result in overdosing when dealing with certain drugs and/or certain patient populations. The best practice would be to introduce a consistent practice to remove inconsistencies from one compounder to the next.
Needle volume must be taken into consideration when a dispensing pin is attached to a vial of medication.
Examples:
If needle dead space is not considered when compounding a TPN that calls for 0.1 mL of insulin and using an insulin vial spiked with a dispensing pin and changing to a compounding needle with a dead space of 0.1 mL, then the final TPN will not contain any insulin. The amount of insulin in the syringe will simply fill the dead space of the needle. Again, “flushing” the needle by aspiration will deliver all the mediation (plus residual from the syringe hub dead space) to the final product but can result in another compounding error as will be discussed later.
To correct this, the compounder must add 0.1 mL or more in addition to the prescribed dose. In the example above, to get the ordered volume of insulin into the TPN bag, the compounder must pull back to 0.2 mL or more on the syringe. Prior to injecting, the compounder will push out the insulin to the desired calibrated marking ensuring the needle dead space is filled. Essentially, this is "priming the needle" making it a "wet needle". The dose is then accuratly injected without "flushing" the syringe to accuratly instill 0.1 mL of insulin.
“Flushing” the syringe by aspiration can result in another compounding error altogether.
As another example, is a NICU patient requiring 0.1 mL heparin in a solution. If 0.1 mL is pulled from a vial of heparin using a needle-syringe combination (no dispensing pin in this example) and inject into a bag using the same needle, 0.1 mL of heparin is delivered to the bag (leaving an additional 0.1 mL in the needle dead space). However, if the compounder inadvertently “flushes” by aspiration then the 0.1 mL heparin remaining in the needle dead space along with the unknown amount in the syringe tip will be also introduced into the preparation resulting 0.2 mL PLUS what was remaining in the tip of the syringe (more than double the ordered dose).
Therefore “flushing” should be avoided during compounding. The concept is easily understood once explained but by allowing multiple practices to exist increases risk for compounding errors.
There isn’t one correct answer to put a fail-safe practice into place. Each facility must decide what is right for their team and workflow. However, it is suggested that “flushing” syringes by aspiration always be avoided, and the needle overfill volume be introduced for all drugs of 1 mL or less. For consistency, this would include all drugs (even those that would not be of clinically significant impact) as to introduce standardization across compounders at the facility. However, strong consideration should be made for "priming the needle" for NICU, drugs with narrow therapeutic ranges, or any other ingredient that requires extremely precise dosing in small amounts where any small increase or decrease in active ingredient would prove harmful or nontherapeutic (e.g. Eye injections).
In conclusion:
Anytime a needle is added/changed to a syringe that already has a measured amount of drug of 1 mL or less, the needle dead spaced volume should be added (0.1 mL for compounding needles and 0.05 for needles intended for administration). Again, the only way to know the true volume for needle dead space is to contact the manufacture or to test each needle used by the facility (remembering different gauges, needle lengths, and variations between manufactures).
Adopt "priming the needle" to eliminate the “flushing” by aspiration method to deliver all medication to the final compounded preparation to avoid inadvertent addition of drugs with a narrow therapeutic range that could be of clinical significance.
When using a filter needle/straw, "priming the needle" should be observed to avoid "flushing" by aspiration (remembering that filter needles and filter straws will have varying dead spaces and should be researched).
When sending a syringe containing a drug for administration, the needle volume of 0.05 mL should be added to accommodate for the administration needle volume. Nurses must be trained in this process, so they are aware that the overfill volume is intentional and to be used to “prime” the needle. NOTE: If the facility uses a needless IV system, then no needle will be attached, and no extra volume needs to be added. In this situation, only IM or SQ will need extra volume since a needle will be added for administration.
Pharmacist (BCSCP) and the founder of Soigner Solutions (Soigner Universal, LLC). Collectively, Steve has over 20 years experience in sterile compounding of non-hazardous and hazardous preparations (including non-sterile to sterile using API). He is also experienced in non-sterile compounding and has fully developed and implemented all-inclusive compounding programs including USP <800> hazardous drug programs since the release of <800> in 2016. Steve received his Doctor of Pharmacy degree from McWhorter School of Pharmacy and holds two other degrees from Samford University and Université Sorbonne.
Thank you for visiting my site. For more information please visit www.SoignerSolutions.com or email me directly at smilstead@SoignerUniversal.com
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