Perpetual Inventory System: How It Works and How to Reconcile It to Physical Counts

A perpetual inventory system keeps a running record of every item in a supply room: each receipt adds to the balance, each issue subtracts from it, and the balance is always available without counting the shelf. This guide is for supply-room leads, lab managers and buyers who need to know what that record is, why it still has to be checked against a physical count, and what to verify before relying on one. It does not repeat how counting schedules work; see cycle counting inventory for that.

What a perpetual inventory system is

A perpetual (or book) inventory is a continuously updated record of quantities and values on hand. The U.S. Internal Revenue Service describes it in Publication 538 as one of two accepted ways to figure the cost of goods on hand, provided inventory is kept following sound accounting practices. Under that description, inventory accounts are charged with the actual cost of goods purchased and credited with the value of goods used, transferred or sold. Checked 3 November 2026.

In a clinic or laboratory this means a ledger per item or location, fed by receiving and by issue or use. It sits alongside, not instead of, the practices described in inventory management in healthcare and medical supply inventory management.

How the ledger works: receipt, issue, adjustment

• Receipt. Goods are added to the balance when received. IRS Publication 538 counts purchased merchandise as inventory once title has passed, even if still in transit, and excludes goods ordered for future delivery without title.
• Issue. Each item used, transferred or sold is credited out of the balance. Par-based restocking, covered in par levels for medical supplies, reads from this balance.
• Adjustment. A change to the balance that is not a receipt or an issue, for example after a count finds a difference. Publication 538 requires the book amount to be adjusted to agree with the actual inventory.

What to record for each item (lot, expiry, identifiers) is covered in medical inventory tracking.

Perpetual vs periodic inventory

Question	Perpetual	Periodic	Source
How is the record updated?	Per transaction, in real time through barcode, point-of-sale or software	At set intervals, from a count	Bloomreach, “Perpetual vs periodic inventory system”
Is stock known between counts?	Yes, as a recorded balance	No	Bloomreach
Is a physical count still needed?	Yes, at reasonable intervals, and the book amount is adjusted to agree	Yes, it is the basis of the record	IRS Publication 538; Bloomreach

Why physical counts are still required

The recorded balance is a claim about the shelf, not a measurement of it. IRS Publication 538 states that a physical inventory must be taken at reasonable intervals and the book amount adjusted to agree with it. Commercial explainers add that occasional manual audits remain necessary to catch theft, damage and missing items.

Award-funded equipment has its own rule. Under 2 CFR 200.313(d), a physical inventory of equipment must be conducted at least once every two years and the results reconciled with the property records, which include description, serial or other identification number, funding source, title holder, acquisition date, cost, location, use, condition and disposition. This applies to equipment under federal awards, not to consumables.

Reconciliation and adjustment controls

A 2005 audit by the New York City Comptroller of inventory controls at Bellevue Hospital Center (report MG03-179A, 21 April 2005) is one dated public example. It found that Central Supply Stores kept no perpetual inventory records and counted only at fiscal year-end, and recommended keeping perpetual records, preferably on a computerized system, and following the written count procedures when taking the physical count. Separately, for the hospital’s noncontrolled drugs in the Pharmacy Department, the audit reported that discrepancies were not investigated. This is one audit of one hospital in 2005, not a statement about hospitals generally.

Taken with the IRS wording on adjusting the book amount to agree with the physical count, four controls are common in practice:

1. Investigate every difference between the count and the recorded balance before adjusting.
2. Keep the person who maintains the records separate from the person who counts stock.
3. Document, review and approve each adjustment.
4. Follow written count procedures.

Where loss is the concern, see inventory shrinkage.

What to verify when choosing a system

These are questions to put to a software vendor or supplier, drawn from the sources cited below rather than from a regulatory requirement; none of them sets a mandated variance threshold or requires a particular system.

• Does it charge on receipt and credit on issue, with a history for each item?
• Can adjustments be entered only with a reason and an approval, and are they logged?
• Can count results be entered and compared with the recorded balance without overwriting it?
• Can it separate record-keeping roles from counting roles?

For software options and selection topics see medical inventory management software.

Common mistakes

• Treating the recorded balance as proof of stock and skipping counts.
• Adjusting the balance to match a count without investigating the difference.
• Letting the same person maintain the record and perform the count.
• Applying the two-year equipment rule of 2 CFR 200.313 to consumables.

FAQ

What is a perpetual inventory system?

A continuously updated record in which purchases are charged to inventory and goods used, transferred or sold are credited, so the balance is known at any time (IRS Publication 538).

Does a perpetual inventory system replace physical counts?

No. IRS Publication 538 requires a physical inventory at reasonable intervals, with the book amount adjusted to agree.

How does it differ from periodic inventory?

Periodic systems update from counts at set intervals, so stock is unknown between counts; perpetual systems update per transaction. The difference between cycle counts and a full count is explained in cycle counting inventory, and ownership arrangements are compared in consignment vs vendor-managed inventory.

Sources

• IRS, Publication 538, Accounting Periods and Methods, “Perpetual or Book Inventory” and “Physical inventory”: irs.gov/publications/p538 (checked 3 November 2026).
• eCFR, 2 CFR 200.313, Equipment: ecfr.gov (checked 3 November 2026).
• New York City Comptroller, Audit Report on the Inventory Controls of Bellevue Hospital Center over Noncontrolled Drugs and Medical and Surgical Supplies, MG03-179A, 21 April 2005: comptroller.nyc.gov (checked 3 November 2026).
• Bloomreach, Perpetual inventory system vs periodic inventory system (definitions only): bloomreach.com (checked 3 November 2026).