Medical gas calculation: how to bill correctly

Medical gases are present in practically every department of a hospital: from the emergency room to the ICU, from the operating room to the ward. Even so, calculating medical gases for billing still raises frequent questions among teams, and the consequences of inaccurate recording go straight to the institution’s financial results: denials, undercharging and lost revenue that could be avoided with more structured processes.

Understanding how these supplies are regulated, how they appear on the hospital claim and what the documentation requirements are is the starting point for billing with confidence and keeping disputes from health plans to a minimum.

What medical gases are and how Anvisa classifies them

Medical gases are gaseous substances intended for the prevention, diagnosis or treatment of diseases in humans, or used to restore, correct or modify physiological functions. Because of their therapeutic purpose, Anvisa (National Health Surveillance Agency) classifies them as medicines, which imposes a specific set of regulatory requirements on healthcare institutions and manufacturers.

This classification is based on RDC No. 69/2008, which establishes Good Manufacturing Practices for Medical Gases and requires every batch to be tested and released before sale, with traceability guaranteed from manufacturing to the point of use.

In 2024, Anvisa published the sector’s new regulatory framework, RDC No. 870/2024, which updated the requirements for notification, registration and post-registration changes, tightening control over which gases can be sold on the market for medical purposes.

In the hospital setting, the most widely used gases are:

• Medical oxygen: the main therapeutic gas, used in oxygen therapy, ventilatory support and surgery.

• Medical compressed air: drives mechanical ventilators, neonatal incubators and pneumatic surgical equipment.

• Nitrous oxide: an anesthetic and analgesic agent, used in surgical and obstetric procedures.

• Carbon dioxide (CO2): used in laparoscopies and other minimally invasive procedures.

For billing, classification as a medicine has a direct implication: the hospital must prove use of the gas with the same consistency required for any other medicine administered to the patient.

Prescription, recording of use in the medical record and consistency between what was prescribed and what was charged are the three pillars that support a charge that cannot be disputed.

How medical gases appear in hospital billing

In the TISS standard of the ANS (Brazil’s National Supplementary Health Agency), medical gases are classified under code group 18 of the TUSS Table, called “Terminology of Daily Rates, Fees and Medical Gases.” This group brings together hospital expense items that are neither medical procedures nor medicines for internal use, but that make up the patient’s claim and must be coded correctly for the health plan to process payment.

In practice, billing takes four main elements into account:

Type of gas

Oxygen, compressed air, nitrous oxide or another gas used, each with its own TUSS code.

Gas therapy modality

Oxygen therapy by nasal cannula, Venturi mask, non-rebreather mask, mechanical ventilation, among others. The modality affects the prescribed flow and, in some contracts, the unit price charged.

Quantity consumed

Calculated from the prescribed flow in liters per minute (L/min) and the duration of use in hours. The basic formula is total volume (L) = flow (L/min) x duration (min).

Contract price

Each health plan may have its own price table for gases, defined in its contract with the hospital. Some payers pay per liter consumed, others per hour of use, and still others include the gas in daily rate or procedure packages.

A practical example: a patient on oxygen therapy by nasal cannula at 3 L/min for 12 hours. The volume consumed is 3 x 720 minutes = 2,160 liters of oxygen. This volume is what should support the charge, respecting the unit of measure and the price agreed in the contract with the health plan.

Some contracts include oxygen in the nebulization fee or the ICU daily rate. Charging for the gas separately in these cases is one of the most frequent causes of denial, identified in Table 38 of the TISS standard as “Charge for oxygen included in the specified nebulization fee.”

That is why, before entering any gas item on the claim, the billing team needs to check whether the payer’s contract provides for this charge as a stand-alone item or whether it is already built into another item.

The role of nursing records in the calculation

If the medical prescription is the starting point for billing medical gases, the nursing record is what turns that prescription into evidence of use. Without it, the charge is vulnerable to any dispute from the health plan, regardless of whether the gas was actually administered.

For billing to hold up, the medical record must contain at least three pieces of information:

• A complete medical prescription: type of gas, flow in liters per minute and expected duration of use. The prescription is the clinical authorization that justifies the charge. The absence of this document directly triggers the denial “Charge for oxygen therapy without a medical prescription,” listed in Table 38 of the TISS standard.

• Record of start and end: the nurse must note the time the gas was started and the time it was stopped or changed. This information makes it possible to calculate the actual time of use and, consequently, the volume consumed.

• Any flow adjustments: if the physician changed the prescribed flow during care, the change must be recorded in the medical record with the date, time and signature. Billing based on the original flow when it was reduced is an inconsistency that the health plan’s auditor will easily spot.

With this data documented, the calculation of the volume consumed follows the logic already described: flow (L/min) x time of use (min). If a patient used oxygen at 2 L/min for 8 hours and then had the flow adjusted to 3 L/min for another 4 hours, the correct calculation is:

• First period: 2 x 480 = 960 liters

• Second period: 3 x 240 = 720 liters

• Total to bill: 1,680 liters

The nurse auditor’s role in this process is to check, before the claim is closed, whether the care records are consistent with what is being charged. Any discrepancy between what was prescribed, recorded and billed is an opening for a denial by the health plan, and an avoidable loss for the hospital.

Main causes of denials for medical gases

Table 38 of the TISS standard lists the reasons health plans can dispute charges related to medical gases. Knowing these reasons allows the billing team to act preventively, before the claim is submitted.

The most frequent denials for this item are:

A medical gas denial is rarely isolated. It usually occurs together with other inconsistencies on the same claim, which increases the financial impact and makes it harder to identify the root cause. That is why mapping denials by type and frequency is the first step toward a more robust billing process.

How to check the contract with the health plan

The contract signed between the hospital and each health plan is the document that defines the billing rules for medical gases for that specific payer. The terms often vary: one health plan may pay per liter consumed, another per hour of use, and a third may include certain gases in daily rate or operating room packages.

To avoid denials due to contract discrepancies, the billing team should map, for each payer:

• Which gases are covered as stand-alone billable items and which are included in packages.

• The unit of measure used for payment (liter, hour, fraction of an hour).

• Quantity limits by type of care, where they exist.

• Whether prior authorization is required for charges above a certain volume.

This check is especially critical for prolonged hospital stays, mechanical ventilation and surgeries using nitrous oxide, where the volume consumed can be high and the chance of a dispute is greater.

Hospitals with multiple payers deal with different rules for the same item. Keeping this information up to date and accessible to the billing team reduces systematic errors that repeat month after month, payer after payer.

Well-billed medical gases make a difference in the revenue cycle

Medical gas denials have a characteristic that makes them especially harmful to the hospital: they are recurring. A process with a documentation or contract-check failure generates the same loss month after month, for every patient who uses oxygen, compressed air or another gas during their hospital stay.

With a high volume of care, this cumulative impact represents a significant loss in the revenue cycle, often invisible to management because each individual occurrence seems small.

Rivio uses artificial intelligence to manage the entire hospital revenue cycle, increasing revenue and operational efficiency. From audit to payment, the technology analyzes clinical records, cross-checks information against hospital claims, identifies and corrects inconsistencies before submission, and manages denial appeals automatically.

FAQ: frequently asked questions about medical gas calculation

Are medical gases billed as a medicine or as a hospital fee?

They are billed as a hospital expense item, within group 18 of the TUSS Table, called “Daily Rates, Fees and Medical Gases.” Although Anvisa classifies them as medicines for regulatory purposes, in billing to health plans they follow TUSS’s specific coding for gases, separate from medicines administered orally or parenterally.

Can oxygen used in nebulization be billed separately?

It depends on the contract with the health plan. With many payers, oxygen used in nebulization is already included in the procedure fee. Charging for the gas separately in these cases generates the denial “Charge for oxygen included in the specified nebulization fee.” A contract check is mandatory before entering the charge.

What happens if the gas was used but not formally prescribed?

The charge is vulnerable to a denial for lack of a medical prescription, one of the most frequent reasons for disputes in this category. Beyond the financial loss, the lack of a prescription is a clinical irregularity. The right approach is to regularize the documentation before the claim is closed or, preventively, to set up a workflow that prevents the gas from being administered without a prescription recorded in the medical record.

How do you calculate oxygen consumption for billing?

The calculation starts from the medical prescription: flow in liters per minute (L/min) multiplied by the time of use in minutes. Example: 2 L/min for 6 hours = 2 x 360 = 720 liters. If the flow was adjusted during care, each period must be calculated separately and the volumes added together. The total is what should be entered on the claim, respecting the unit of measure in the contract with the health plan.

Which TUSS code should be used for medical gases?

Medical gases are in group 18 of the TUSS Table. Each type of gas and mode of use has its own code. The correct code depends on the type of gas (oxygen, compressed air, nitrous oxide, CO2), the mode of administration and what is set out in the contract with each health plan. Checking the current TUSS table before each entry is the safest practice to avoid denials for invalid or inactive codes.