2.5 High-Level Disinfection (HLD)

High-level disinfection (HLD) eliminates all microorganisms — bacteria, viruses, fungi, and mycobacteria — except high numbers of bacterial spores. It is the minimum reprocessing requirement for semi-critical items: devices that contact mucous membranes or non-intact skin but do not enter sterile tissue. Under the Spaulding classification, examples include flexible gastrointestinal endoscopes, bronchoscopes, laryngoscope blades, and respiratory therapy equipment.

(Critical items that enter sterile tissue require sterilization, not HLD; non-critical items that touch only intact skin need low- or intermediate-level disinfection.)

The golden rule still applies: HLD only works on a clean device. Soil and biofilm physically shield organisms from the chemical, so meticulous cleaning must come first.

Common HLD Agents

The CRCST exam expects you to know that OPA acts faster than glutaraldehyde and gives off fewer fumes, but stains tissue and surfaces gray, and that glutaraldehyde must be activated before use and is a potent respiratory irritant.

Manual HLD Process (step by step)

1. Clean thoroughly first — HLD fails on a soiled device.
2. Test the solution with MEC test strips before this immersion; the concentration must be at or above the Minimum Effective Concentration.
3. Completely immerse the device; fill all channels and lumens — no part may be above the liquid.
4. Maintain immersion for the full labeled contact time at the labeled temperature.
5. Rinse thoroughly with sterile or filtered water (tap water only when the IFU permits) to remove all chemical.
6. Dry with forced air or an alcohol flush followed by air drying per IFU — residual moisture supports waterborne organisms such as Pseudomonas.
7. Document date, time, chemical, contact time, MEC result, temperature, and technician initials.

Critical points

• MEC is tested before EACH use, not once per day — a frequently missed exam point.
• If MEC is below minimum, discard the solution even if the calendar use-life has not expired.
• A solution can fail either when MEC drops or when the use-life days expire — whichever comes first.
• Never shorten contact time; trapped air bubbles in lumens block contact and must be purged.
• Inadequate rinsing leaves residue that causes chemical colitis or burns to mucous membranes.

Automated Endoscope Reprocessors (AERs)

AERs automate HLD for flexible endoscopes and give more consistent, reproducible results than manual processing.

Advantages: standardized temperature, time, and concentration; automated channel flushing reaches every internal surface; built-in rinse cycles ensure removal; electronic cycle records; far less chemical exposure to staff.

The endoscope reprocessing sequence:

1. Bedside (POU) precleaning — wipe the insertion tube; flush channels with enzymatic detergent.
2. Leak testing — detect holes or channel damage before immersion.
3. Manual cleaning — brush every channel and the valves/ports.
4. Place in the AER — connect all channel adapters precisely.
5. AER runs wash, HLD, rinse, and (often) an alcohol/forced-air step.
6. Dry and store hanging vertically in a ventilated cabinet per IFU.

Skipping leak testing risks pushing fluid into the scope's interior, causing costly damage and creating an uncleanable reservoir for organisms.

Chemical Safety for HLD

• PPE: chemical-resistant gloves, face shield or goggles, and a fluid-resistant gown.
• Ventilation: use a fume hood or vapor-controlled station; covered soak basins limit fumes.
• Exposure limits: the ACGIH/NIOSH ceiling for glutaraldehyde is about 0.05 ppm; OPA has no formal OSHA PEL but exposure must be minimized.
• Spill response: know neutralizer (e.g., glycine for glutaraldehyde) and spill-kit locations.
• SDS: keep current Safety Data Sheets for every HLD chemical.

Worked Scenario and Traps

Scenario: A technician soaks a colonoscope in OPA for 12 minutes but skipped leak testing and channel brushing, and reused the solution because "the calendar says 10 days left." Two failures: HLD on an unbrushed scope leaves biofilm that the chemical cannot penetrate, and MEC was not verified — the solution may already be below 0.3%. The scope is not safely disinfected.

Common traps: confusing HLD with sterilization (HLD does not reliably kill high numbers of spores); testing MEC daily instead of before each use; trusting use-life dates over MEC strips; and inadequate rinsing that injures the next patient.

A further nuance the exam may probe is the difference between HLD and liquid chemical sterilization. Some of the same chemistries — glutaraldehyde at extended contact times, or peracetic acid systems — can sterilize, but only with prolonged immersion under controlled conditions, and the resulting device is wet and cannot be wrapped to maintain sterility over time. For that reason, liquid chemical sterilization is reserved for heat-sensitive critical devices that must be used promptly, and it is not equivalent to terminal steam or low-temperature sterilization.

Storage after HLD is equally important: a properly disinfected endoscope that is stored damp or coiled tightly can become recontaminated, so scopes are hung vertically, uncapped, in a ventilated drying cabinet, and channels are dried with forced air. Reprocessing failures with flexible endoscopes have caused real patient-infection outbreaks, which is why every step — precleaning, leak testing, brushing, MEC verification, immersion time, rinsing, drying, and documentation — must be performed exactly as written, every time.