HLD Test Strips and MEC Monitoring: How to Test a High-Level Disinfectant
At LabSupplies.com, we treat high-level disinfectant test strips as the one step in a disinfection workflow that proves the chemistry still works. A strip verifies that a reusable HLD solution holds its minimum effective concentration (MEC) before an instrument goes in. The rule is simple: test before each use, and use the strip validated for your specific solution. Strips are matched to a named product, not to a chemistry class, so an OPA strip and a glutaraldehyde strip are not interchangeable, and neither are two glutaraldehyde strips rated at different concentrations.
What Minimum Effective Concentration Measures
A reusable HLD solution is diluted every time a wet instrument enters it, and the active ingredient is consumed as it works. The solution can fall below the concentration required for high-level disinfection long before its stated reuse life expires. The MEC test measures the active concentration at the moment of use, not how many days are left on the label.
CDC's guidance on chemical disinfectants notes that data suggest 1.0% to 1.5% glutaraldehyde is the minimum effective concentration for glutaraldehyde solutions sold at more than 2%, that the solution should be considered unsafe when testing shows it has fallen below the product's MEC, and that strip results should be documented.
Reuse Life Is a Ceiling, Not a Guarantee
A "28-day" glutaraldehyde is not certified to work for 28 days. Twenty-eight days is the maximum reuse period; in a busy endoscopy suite the solution can drop below MEC within days. The label sets the ceiling; the strip sets the actual pass or fail. A solution that fails is discarded and replaced — never topped off with fresh concentrate, because adding solution restores volume without reliably restoring active concentration across the bath.
Three Clocks You Have to Track
- Solution reuse life: days from activation of the disinfectant, typically 14 or 28 depending on the product.
- Strip bottle open-life: days from the date you first open the bottle, printed on it and commonly three to six months.
- Strip shelf life: the expiry printed for bottles that have never been opened.
CDC guidance is to date the bottle of strips when it is opened and use it only for the period shown on the bottle. Most bottles carry a "date opened" field for exactly this reason — write on it. A bottle that looks new but was opened five months ago will still change color and still tell you nothing.
Match the Strip to the Solution
Every strip is validated for a named disinfectant. These are common pairings with their manufacturer numbers, so you can confirm the strip your disinfectant's label specifies:
| Strip | Reads | Validated for | Manufacturer # |
|---|---|---|---|
| MetriTest 1.8% | Glutaraldehyde, 1.8% | MetriCide 28, MetriCide Plus 30 (28-day) | 10-304 |
| MetriTest 1.5% | Glutaraldehyde, 1.5% | MetriCide (14-day) | 10-303 |
| MetriCide OPA Plus indicator | OPA | MetriCide OPA Plus | 10-602A |
| Rapicide OPA/28 test strips | OPA | Rapicide OPA/28 | ML02-0137 |
| Revital-Ox RESERT R60 strip | Hydrogen peroxide | Revital-Ox RESERT HLD | PCC054 |
| Tristel ULT test strips | Chlorine dioxide | Tristel ULT | 45-10 |
Two glutaraldehyde strips that are not substitutes: MetriTest 1.8% and MetriTest 1.5% look nearly identical and differ by one digit in the manufacturer number. They read different thresholds for different reuse periods. Using the 1.5% strip on a 28-day solution can pass a bath that is already below the concentration that solution requires. If your department runs both, separate them and label the shelf.
How to Run the Test
Dip and read times are product-specific, so follow the strip's own instructions. In general terms:
- Confirm the bottle is in date, within its open-life, and validated for this solution.
- Dip one strip for the specified time. Do not swirl it or leave it in longer than stated.
- Hold it horizontally so solution does not run across the pad.
- Read at the specified time against the color chart on that bottle.
- Record the result, date, solution lot and operator, then discard the strip.
- On a fail, discard the solution. Do not top off and do not retest the same bath.
Indicators for Automated Ultrasound-Probe HLD
Automated probe systems replace the open bath, but they still need a chemical indicator to confirm each cycle. We stock the consumables for two of them:
- trophon2 uses Sonex-HL, a 35% hydrogen peroxide disinfectant, to disinfect ultrasound probes in a closed chamber. We stock the trophon chemical indicator (N05003, box of 300), the trophon disinfectant (N05002, 80 mL, case of 6) and trophon cleaning wipes (N00112-US).
- Tristel ULT is a chlorine dioxide-based system for probes. The Tristel ULT test strips (45-10, 50 strips) are semi-quantitative chemical indicators made for use with it.
As with immersion strips, use the indicator the system's instructions name, and record each result with the probe and cycle.
Handling and Disposal
MEC testing puts staff over an open bath of active disinfectant. OSHA has no permissible exposure limit for glutaraldehyde, but the ACGIH ceiling threshold limit value is 0.05 ppm, and glutaraldehyde is a strong irritant and sensitizer. OSHA's hospital eTool calls for good ventilation and local exhaust, covered baths, and gloves impervious to glutaraldehyde such as nitrile or butyl rubber. Where eyes may be exposed to corrosive materials, OSHA 29 CFR 1910.151(c) requires facilities for quick drenching or flushing within the work area.
Spent solution should be neutralized before disposal where your wastewater authority requires it. Aldex aldehyde neutralizer (AMS1010) is labeled for glutaraldehyde up to 4% and 10% formalin; for OPA, follow the neutralization step on the product's own label.
Equipment for the Testing Station
| Station step | What it needs | What we stock |
|---|---|---|
| Pre-clean | Enzymatic detergent and brushes sized to each lumen | Enzymatic instrument detergents and instrument cleaning brushes |
| Immersion | A covered tray deep enough to submerge every channel | ProSoak instrument soaking tray (75159), Sklar soaking tray 4 × 7 × 13 in (10-3054) |
| Contact time | A timer per bath, so no tray comes out early | Heathrow Lab Alert Triple Timer (HS24490M), three independent channels |
| Verification | The strip or indicator the disinfectant's label names | Tristel ULT test strips (45-10), trophon chemical indicator (N05003) |
| Protection | Impervious gloves and eye protection | Nitrile exam gloves |
| Emergency | An eyewash within the work area | Global Industrial portable eyewash, ANSI Z358.1 (708606) |
| Disposal | A neutralizer matched to the chemistry | Aldex aldehyde neutralizer (AMS1010) for glutaraldehyde |
| Spills | A kit at the station | Spill kits |
Frequently Asked Questions
How often should HLD test strips be used?
Before each use of a reusable high-level disinfectant. The strip measures the active concentration at that moment, and a solution can fall below its minimum effective concentration well before its labeled reuse life ends. Record every result.
What is the minimum effective concentration of glutaraldehyde?
CDC notes that data suggest 1.0% to 1.5% glutaraldehyde is the minimum effective concentration for solutions sold at more than 2%. The exact pass threshold is the one on the strip the disinfectant's label specifies, which is why strips are matched to a named product.
Can I use any glutaraldehyde test strip?
No. Strips are validated for a named solution and read a specific threshold. A strip for a 14-day solution can pass a 28-day bath that is already below the concentration that solution requires. Use the strip the disinfectant's label names.
How long are HLD test strips good after opening?
For the period printed on the bottle, counted from the date you open it. CDC guidance is to date the bottle when opened. Open-life commonly runs from about three to six months depending on the product, and it is separate from the unopened expiry date.
What should I do if a high-level disinfectant fails the test strip?
Discard the solution and prepare fresh. Do not top it off with concentrate and do not retest the same bath hoping for a pass. Document the failure, the date and the operator.
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Send your disinfectant and your current testing routine to support@labsupplies.com and we will check the station against it.
Related Guides
- High-Level Disinfection: Choosing a Method and Setting Up the Station
- Enzymatic Instrument Detergents
- Instrument Cleaning Brushes: Sizes and Bristles
- Instrument Reprocessing Kit Checklist
- Lab PPE Requirements
Sources and Technical References
- Chemical Disinfectants, Guideline for Disinfection and Sterilization in Healthcare Facilities, Centers for Disease Control and Prevention — cdc.gov
- Hospital eTool: Glutaraldehyde, Occupational Safety and Health Administration — osha.gov
- 29 CFR 1910.151 — Medical Services and First Aid, Electronic Code of Federal Regulations — ecfr.gov
— By the LabSupplies.com Technical Team