Product loss in fill-finish is one of the biggest drivers of manufacturing yield. Every milliliter retained in vessels, filters, tubing, sampling, or filling equipment reduces the number of finished units that reach patients. While each individual source of loss may appear small, together they can consume a significant percentage of a batch, especially for biologics, oligonucleotides, cell and gene therapies, and other high-value drug products.
This article maps where drug product is lost across sterile fill-finish, and how to recover it.
What is Product Loss in Fill-Finish?
Product loss in fill-finish is the portion of a drug product batch that does not become an acceptable finished unit. Loss can occur when product remains in vessels, filters, tubing, pumps, and filling equipment; when units are consumed for testing; or when filled containers are rejected during inspection.
Some product loss is unavoidable. Sterile filters retain a certain amount of liquid, analytical testing requires finished units, and each container may need to be filled above its labeled volume to ensure the required deliverable volume. Other losses can be reduced through process and equipment design. Oversized filters, excessive tubing, destructive weight checks, poor vessel drainage, inconsistent filling, and avoidable cosmetic rejects can all increase product loss in fill-finish.
These losses matter most when batch volume is limited or the drug product is expensive, difficult to manufacture, or intended for a small patient population. For high-value biologics, oligonucleotides, orphan drugs, and gene therapies, recovering even a few additional milliliters can produce more finished units, protect clinical supply, and reduce the need for additional drug substance manufacturing.
The sources of product loss vary across the fill-finish process:
| Fill-Finish Step | Typical Sources of Product Loss |
| Formulation and Compounding | Vessel and surface hold-up |
| Sterile Filtration | Filter and flow-path hold-up |
| Sterile Filling | Line priming, weight checks, end-of-batch losses, and overfill |
| Quality Controls | Visual inspection rejects and sampling for in-process, release, stability, and retains |
Understanding where these losses occur allows sponsors and fill-finish manufacturers to focus engineering efforts on the steps that have the greatest effect on overall batch yield.
Where product loss occurs in: Formulation and Compounding
Vessel and surface hold-up
Drug product wets every surface it touches:
- The compounding vessel
- The impeller or mixer
- Dip tubes
- The transfer set feeding the filter
The residual liquid retained on those surfaces when the vessel drains is hold-up volume, and it does not scale with batch size, but with wetted surface area. That distinction becomes critical at small volumes. A 200 L batch may lose a fraction of a percent to vessel hold-up, but a 2 L batch of an oligonucleotide or viral vector, processed in comparable hardware, can lose several percent to the same surfaces.
How to reduce hold-up during formulation
Select the appropriate vessel geometry. How a vessel drains determines how much of that film you recover. Conical or dished bottoms with the outlet at the true low point drain cleanly, but flat-bottomed or side-drained vessels leave a heel. Glass and stainless vessels that must be siphoned or pumped from the top strand more product than single-use bags that drain by gravity from the bottom.
Where drug product is lost in: Sterile Filtration
Sterile filtration is usually the single largest source of unrecoverable loss during the manufacturing process.
Filter hold-up
Filter hold-up is often one of the largest contributors to product loss in fill-finish because product remains inside the membrane, housing, and tubing after filtration. Proper filter sizing helps reduce this loss.
How to reduce filter hold-up
Right-size the filter. Matching the most effective filtration area to the batch volume and the product’s fouling behavior (the accumulation of particles or substances on the filter) will reduce hold-up significantly. A Vmax or Pmax study should be conducted to identify the best-fit membrane type and size for a given drug product lot.
Flow-path hold-up
Filtration pumps bulk solution from a source vessel through tubing, across two filters, and into a receiving vessel or surge bag. Product is retained on every wetted surface along that path, with most of it retained in the filter housings and downstream tubing.
How to reduce flow-path hold-up during filtration
Blow down the filtration line. A low-pressure gas purge after filtration pushes product through the assembly more completely. The greatest recovery here typically comes from clearing the tubing between the two filters.
Where drug product is lost in: Sterile Filling
Sterile filling also loses product during setup, filling, and end-of-fill activities.
Line priming
Before the first container is filled, the fill line assembly must be purged of air and brought to a stable, repeatable fill weight. Drug product is pumped through the assembly to displace air ahead of the fill—a step called priming. To confirm the line is free of air and bubbles, some manufacturers require a set prime volume from each fill needle, typically 5–10 mL per needle. That volume is discarded, and it scales with the number of needles on the line.
How to reduce line priming loss
Rescue dosing. On fill lines with rescue-dosing capability, line priming losses can be eliminated. The system weighs each dispensed container and directs the line to top up any that fall short, bringing them into the acceptable fill-weight range rather than rejecting them.
Weight checks
Weight checks run before, during, and at the end of the fill to keep fill volumes in range. Each needle must pass consecutive checks before the fill starts, confirming stable, repeatable weights. During the run, a bracketing weight checks catch any drift, and a final check on the last container bookends the lot.
How to reduce loss from weight checks
Non-destructive weight checks. Some manufacturers, including Argonaut, offer 100% non-destructive weight checks on their filling lines that eliminate product loss from weight checks. With this capability, comes the potential to top up low-filled units to bring them into range (i.e. rescue dosing), which is also a capability at Argonaut.
End-of-batch losses
Filling is often stopped when air reaches the pump, because entrained air destroys fill-weight accuracy. Whatever product remains downstream — in the manifold, tubing, and needles — is typically dispensed to waste.
How to reduce end-of-batch losses
Rescue dosing. On fill lines with rescue-dosing capability, end-of-batch losses can be almost entirely recovered. Rescue dosing non-destructively weighs each dispensed container and tops up any that fall below specification, bringing them into the acceptable fill-volume range. Because every container is verified to contain the correct volume the batch can continue to run, even as pump accuracy is lost.
Overfill
Overfill is deliberate loss. To guarantee the labeled deliverable volume after withdrawal (per USP requirements for injections), every unit is filled slightly above label claim.
How to reduce overfill
Accurate fill pumps. The size of that overfill is set by fill-weight variability: a filler with a tight fill-weight distribution (expressed as coefficient of variation, or CV) can safely target a smaller overfill than a variable one. Across a large batch, trimming the overfill target can increase yield.
Where drug product is lost in: Sampling and Analytical Testing
Finished-unit consumption affects final batch yield, and sterile filling manufacturers should optimize testing where possible to reduce sampling requirements.
In-process samples
Bioburden (pre- and post-filtration), appearance, and fill-weight checks all draw product. Non-destructive in-process weight checks — weighing containers before and after filling rather than emptying sacrificial units — remove one recurring sampling loss entirely.
Release testing
Sterility, endotoxin, potency, particulate, and identity testing each consume finished units. Because these quantities are largely fixed by method and USP guidelines, they weigh far more heavily on a 500-unit orphan batch than on a 50,000-unit commercial one.
Stability studies
Every timepoint, at every storage condition, across the full shelf-life program pulls units, and for a first-in-class biologic the protocol can be extensive.Â
How to reduce sampling
Non-destructive testing. Using non-destructive test methods will reduce sampling requirements, as it will allow one unit to undergo multiple analytical tests. The greatest impact will be through using a non-destructive container closure integrity test (CCIT), which FDA guidance permits in place of sterility testing during stability studies. This dramatically reduces the number of required units to be sampled for stability.
Cosmetic rejects. Every product lot undergoes 100% visual inspection after fill-finish. Some units are rejected for cosmetic defects, such as scratches or scuffs. Because these units are pulled from the lot regardless, they’re well suited for destructive quality testing: the cosmetic flaw won’t influence lab results and using them improves overall batch yield.
Planning. Strategic resource utilization can reduce the number of units required for analytical testing, especially for stability programs. Planning minimal-but-sufficient pull schedules and using small-volume, low-sample-count analytical methods directly protects inventory.
Designing a Fill-Finish Process That Minimizes Product Loss
Product loss in fill-finish is resolved by cumulative design choices. There are several technologies and capabilities that will consistently improve drug product yield:
- Isolator-based filling improves sterility assurance and reduces particulate rates which in-turn reduces reject rates and improves batch yield.
- Non-destructive weight checks, especially 100% and real-time checks will ensure fill volumes remain in range without consuming product.
- Rescue dosing will reduce loss by bringing low-filled units into range and allow the fill line assembly to be emptied completely at the end of the fill.
- High-precision pumps reduce overfill, allowing sponsors to get more units out of the same batch volume.
- Minimal to no glass-to-glass contact fill line designs will reduce the incidence of cosmetic rejects and protect batch yield.
There are also many strategies that help improve yield, regardless of facility or equipment:
- Identifying areas of loss. Estimating the volume of loss a manufacturer expects to have at each step will help identify where to engineer out product loss before filling begins.
- Match equipment to batch scale. Identifying the best-fit filters, reducing tubing lengths and sizes, optimizing the vessel to improve drainage, or using fewer fill heads are just some of the ways that equipment can be scaled to the batch size to reduce product loss.
- Use high-quality components to reduce cosmetic reject rates.
- Build in recovery via validated blow-down steps to turn filter and line hold-up from loss into product.
- Reduce sampling burden by design. Non-destructive weight checks, small-volume assays, and lean stability plans preserve finished units.
- Tighten fill accuracy via using smaller ID tubing to reduce fill-weight variability and overfill.
The most effective time to engineer out product loss in fill-finish is before the first vial is filled, which makes your choice of fill-finish partner one of the highest-leverage yield decisions you’ll make.
How Argonaut Reduces Product Loss in Fill-Finish
At Argonaut Manufacturing Services, we know every milliliter matters. Our fill-finish operations are built around this principle. Every detail from facility layout and equipment selection to filtration, filling, and container handling is designed to maximize yield without compromising quality or scalability.
Our capabilities include:
- Isolator-based filling for high sterility assurance and lower particulate-driven rejects
- Minimal-to-no glass-to-glass contact to reduce cosmetic rejects that erode yield
- 100% non-destructive weight checks with rescue dosing – every filled container is verified, and low-filled units are brought into range
- High-precision fill pumps that hold a tight fill-weight distribution, allowing smaller overfill targets
- Single-head filling available on all lines to minimize line loss for small batches
- Systems engineered to reduce and recover hold-up during filtration and filling
- Low visual inspection reject rates as a result of controlled, consistent manufacturing operations
- Minimal line-loss – we can reduce total line loss to as little as 25 mL on each of our filling lines
Argonaut has experience filling drug product lots as small as 200 mL, and in one 500-vial fill we achieved 99% of theoretical batch yield.
Download the Every Milliliter Matters whitepaper for the full case study.
Evaluating Fill-Finish Partners
If you are a sponsor qualifying a fill-finish partner, ask questions that probe the whole manufacturing process:
- Do you model hold-up and mass balance before selecting equipment?
- How do you design fill programs to improve yield?
- What recovery steps do you run after filtration and at end of filling?
- Do you offer non-destructive in-process weight checks and rescue dosing? If not, how do you reduce product loss from these checks?
For high-value biologics, orphan drugs, gene therapies, and oligonucleotides — products where the batch may be the entire supply and every milliliter carries outsized value — improving yield should be a major focus of your fill-finish partner.Â
FAQs
1. What causes product loss in fill-finish?
Product loss is caused by many factors:Â
- Adsorption and adhesion to product-contact materials (e.g. compounding vessels, mixing surfaces, tubing, etc.)
- Hold-up in sterile filters and transfer tubing
- Line priming and end-of-batch losses during filling
- Destructive weight checks
- Visual inspection rejects
- Deliberate overfill
- Finished units consumed by in-process, release, stability, and retained sampling
2. How much product loss is typical in fill-finish?
It varies widely with batch size and process design, but larger commercial batches may lose only a few percent, while small high-value batches can lose double digits, largely because hold-up and sampling losses are fixed by surface area and test methods, not by batch volume, so they weigh more heavily on small runs.
For an example comparing total product loss between a baseline batch against an optimized process, see Drug Product Loss: Understanding Yield
3. What is filter hold-up, and how do you reduce it?
Filter hold-up is product trapped in the membrane, housing, and tubing after filtration. Because hold-up is roughly proportional to membrane area, right-sizing the filter to the batch (via a Vmax or Pmax study) and adding a validated gas blow-down recovery step are the two most effective ways to reduce it.
4. How does overfill contribute to product loss in fill-finish?
Overfill is the extra volume added to each vial to guarantee the labeled deliverable volume after withdrawal. The required overfill is driven by fill-weight variability, so a filling line with tighter fill accuracy (lower CV) can safely target a smaller overfill, giving less product away on every unit.
5. How does batch size affect product loss in fill-finish?
Smaller batches often lose a larger percentage of their yield during sterile filling. Hold-up volume scales with wetted surface area and sampling scales with fixed test requirements so the same hardware and testing burden consumes a much larger share of a 2 L batch than of a 200 L one.
