When You Calculate Days of Supply Inventory for Field Inventory in Medical Device Do You Include All Inventory Including Surgical Kit Inventory Quantities?

Yes, with a critical condition: include all field inventory that carries cost and can fulfill demand, including surgical kit quantities, but only when you count kit components at the SKU level and avoid double-counting the assembled kit alongside its contents. The short answer is that excluding surgical kits understates coverage and overstates turns. Including them incorrectly inflates supply and distorts replenishment signals. The correct calculation depends on how you define the unit of demand and whether kits are consigned, trunk stock, or staged at a hospital.

Mechanism: How days of supply actually works in field settings

Days of supply (DOS) is a ratio, not a count. The numerator is the quantity of usable inventory on hand; the denominator is average daily demand, usually a trailing consumption rate. In a distribution center, both sides are stable. In field inventory, both sides move. That instability is why surgical kits create so much disagreement among planners.

The core mechanism: DOS = on-hand units ÷ average daily units consumed. If a field representative holds 40 units of a catheter and the territory consumes 2 per day, DOS is 20 days. The formula is simple. The judgment is in what counts as on-hand and what counts as consumed.

Field inventory is unlike warehouse inventory because ownership, location, and usability diverge. Consigned stock at a hospital is physically present but title may rest with the manufacturer. Trunk stock in a rep's car is accessible but may be reserved for scheduled cases. Surgical kits are assemblies of sterile components that may be committed to a specific procedure days in advance. Each category behaves differently in the numerator.

The deeper mechanism is demand mapping. A kit is not a unit of demand; it is a bundle of SKUs that satisfies one procedure. If you treat the kit as one unit, you create a phantom SKU that does not match how components are replenished or how consumption is recorded. The correct approach is to explode kits into their bill of materials and count each component once. This is the single most important technical point in the entire calculation.

A second mechanism is reservation logic. A kit allocated to a Tuesday case is on hand Monday but unavailable to fill Wednesday demand. If you count reserved kits as free supply, DOS looks healthy while fill rates fall. Good field DOS systems subtract committed quantities from available supply before dividing.

Quantified context: why the inclusion decision moves the number

The magnitude of the error depends on kit share of total field value. In many orthopedic and cardiovascular device territories, surgical kits and trays represent a large fraction of on-hand value and a meaningful share of demand. When kit inventory is excluded, DOS can fall by a wide margin; when kits are double-counted, DOS inflates by a similar order. The direction of error is predictable even when the exact percentage is territory-specific.

Consider a simplified example. A territory holds 600 component units as loose stock and 300 component units inside 15 surgical kits, for 900 total component units. Average daily consumption is 30 component units. Correct DOS is 30 days. If you exclude kits, you get 20 days and trigger unnecessary expedites. If you count 15 kits as 15 units plus 600 loose units, you get 615 units and roughly 20.5 days, which coincidentally looks plausible but is wrong for a different reason. If you count kits and their contents, you get 915 units and about 30.5 days, a modest overstatement from double-counting.

The lesson: the arithmetic is unforgiving. Small definitional errors compound across hundreds of SKUs and dozens of territories. Planners who reconcile DOS to financial inventory will catch double-counting quickly, because the physical count and the book count diverge when the same unit is counted twice.

Timing matters too. Surgical kits are often built and staged days before use. A kit assembled on Friday for a Monday case sits in field inventory over the weekend. If your demand denominator uses a 30-day trailing average and your numerator includes weekend staging, DOS will spike every weekend and normalize midweek. Smoothing with a 7-day or 14-day trailing window reduces this sawtooth.

Comparison: treatment of each inventory class

This table is the operational heart of the answer. The rule is one count per physical unit, mapped to the SKU that actually gets replenished. Kits are containers for components, not separate demand units.

Edge cases that break naive calculations

Consignment changes ownership but not physical availability. If a hospital holds consigned stock, that stock can usually fill demand at that site. Excluding it understates local coverage. Including it in enterprise DOS without an ownership flag overstates manufacturer-owned inventory. The fix is a location-and-ownership dimension, not a binary include/exclude decision.

Loaner kits and demo trays are a persistent trap. They are physically present but not for sale and not part of normal supply. Counting them inflates DOS. Excluding them entirely can hide genuine capacity to serve cases. The defensible approach is to exclude them from DOS but track them in a separate availability metric.

Kits built to order behave differently from kits built to stock. Build-to-order kits are effectively committed demand already, so counting them as free supply is doubly wrong. Build-to-stock kits are genuine buffer and belong in the numerator at component level.

Sterilization and reprocessing loops add a time dimension. Reusable instruments may cycle through sterilization and return to availability. If your system treats them as consumed permanently, DOS drifts low. If it treats them as always available, DOS drifts high. Track reprocessing lead time explicitly.

Red-flag warnings:

Counting an assembled kit and its components in the same numerator, which double-counts every unit in the kit. Excluding consigned stock from local availability while including it in enterprise totals, which produces two contradictory DOS figures. Using a demand denominator that includes kit-level procedures while the numerator uses component units, which mismatches units entirely. Leaving expired, quarantined, or recalled stock in the numerator, which inflates coverage with unusable supply. Ignoring reserved units, which makes DOS look healthy right before a fill-rate failure.

Misconceptions and the counterintuitive caveat

The most common misconception is that kits are a separate inventory category that can be excluded for simplicity. In practice, excluding kits systematically understates days of supply and overstates inventory turns, because the demand those kits serve does not disappear when you remove them from the numerator. You get a flattering but false efficiency picture.

The second misconception is that including kits means counting kits as units. It does not. Counting kits as units is the error, not the inclusion. The correct inclusion is at component level, exploded once.

The counterintuitive detail: more complete inclusion can make DOS look worse, not better, in the short run. When you properly add consigned and staged kit components and subtract reserved units, some territories will show lower coverage than before. That is not a regression; it is a correction. Planners who are rewarded for high turns may resist it. The professional caveat is that DOS is a coverage metric, not a performance score, and optimizing it for appearance defeats its purpose.

A subtler caveat: unit-level DOS can look fine while dollar-weighted DOS is poor, because high-cost kit components dominate value. Track both unit DOS and value DOS, and reconcile them to the general ledger.

Practical actions: building a defensible field DOS

Step one: define the demand unit before defining the supply unit. If demand is recorded as procedures, convert procedures to component consumption using the bill of materials. If demand is recorded as component shipments, use that directly. Never mix levels.

Step two: build a single exploded inventory view. Each physical component appears once, tagged with location, ownership, and status. Assembled kits are views over that base, not additive records.

Step three: subtract committed and unusable quantities. Available supply equals on-hand minus reserved minus expired minus quarantined. Divide available supply by trailing average daily consumption, using a 14-day to 30-day window depending on demand volatility.

Step four: set reconciliation thresholds. If the exploded component count and the physical cycle count differ by more than a small tolerance, investigate before publishing DOS. Flag any territory where kit-level and component-level DOS diverge by more than roughly 20 percent as a data-quality issue.

Step five: publish two numbers, not one. Report coverage DOS and available DOS. Coverage includes committed supply; available excludes it. Stakeholders need both to make replenishment decisions.

Step six: review monthly against financial inventory. DOS that cannot be reconciled to the balance sheet is a planning artifact, not a measurement.

Timeline: how a kit moves through field inventory

The timeline shows why a point-in-time count misleads. A territory can look overstocked on staging days and understocked on reconciliation days. Trailing averages and reservation flags smooth these effects.

Limitations and what the calculation cannot tell you

Days of supply is a coverage estimate, not a forecast. It cannot predict demand spikes from a new surgeon, a competitor shortage, or a recall. It cannot resolve ownership disputes in consignment contracts. It cannot substitute for case-level scheduling data when kits are committed far in advance.

The calculation also depends on data quality that many field organizations do not have. If kit bills of materials are inaccurate, exploding kits propagates the error. If consumption is recorded at the procedure level without component detail, the denominator is an estimate. State these limits explicitly when you publish DOS.

Finally, no single threshold fits every device class. High-value, low-volume implants and high-volume consumables require different trailing windows and different tolerances. Calibrate thresholds to demand volatility rather than adopting one universal rule.

This article provides general educational information about inventory measurement methods. It is not financial, accounting, or regulatory advice, and it does not replace the judgment of a qualified inventory controller, finance professional, or regulatory specialist familiar with your specific devices and contracts. Consult a qualified local professional before changing inventory accounting or reporting practices.