2.2 Chain of Infection and Precautions

Infection spreads only if every link in the chain of infection is intact. Break any one link and transmission stops. CMLA I.1.B.1–2 expects you to name the links, match a laboratory assistant's actual tasks to the link they break, and choose the right isolation layer on top of Standard Precautions.

The six links

1. Infectious agent — the pathogen (bacterium, virus, fungus, or parasite) with enough virulence and dose to infect.
2. Reservoir — where the agent lives and multiplies: a patient, a healthcare worker, a fomite (bed rail, tourniquet, keyboard), standing water, or a specimen tube.
3. Portal of exit — how the agent leaves the reservoir: blood from a needle, respiratory droplets, stool, wound drainage, or aerosol from an uncapped centrifuge tube.
4. Mode of transmission — how the agent travels to the next person (contact, droplet, airborne, vehicle, or vector).
5. Portal of entry — how the agent enters the new host: a percutaneous injury, mucous membranes, non-intact skin, or the respiratory tract.
6. Susceptible host — someone who lacks immunity or has a lowered defense (unvaccinated worker, neonate, oncology patient, uncontrolled diabetes).

If any link is missing, infection does not complete. That is why one missed hand-hygiene moment can finish the chain between two otherwise well-run draws.

Modes of transmission in laboratory work

Do not confuse droplet with airborne. A surgical mask is appropriate for droplet precautions. An N95 respirator is required for airborne isolation; it must be fit-tested and user seal-checked each time you don it. Wearing an N95 just in case for a droplet patient is not a substitute for reading the sign, and a surgical mask is not an N95.

Vehicle transmission is easy to miss on exams because it does not look like a door sign. A glucometer used on sequential patients without disinfection, a reused safety lancet, or a multi-dose diluent vial entered with a dirty needle can infect many people from one reservoir. That is still the chain of infection—mode of transmission is the vehicle.

Standard Precautions versus isolation signage

Standard Precautions stay on for every patient, every draw, and every waived test. Transmission-based precautions are posted on the door or flagged in the electronic health record. In a hospital laboratory or phlebotomy context you:

1. Read the sign before you enter.
2. Do not rely on the patient to name the organism.
3. Add the PPE and room controls the sign requires without dropping gloves, hand hygiene, or sharps safety.

Contact signs typically mean gown and gloves for all contact with the patient and the environment (MRSA, vancomycin-resistant enterococci, Clostridioides difficile, norovirus). For C. difficile and norovirus, facility policy usually requires soap and water rather than alcohol-only hand rub because spores and some non-enveloped viruses are not reliably killed by alcohol. Droplet signs mean a surgical mask within close range. Airborne signs mean a monitored airborne infection isolation room (AIIR)—negative pressure, door closed—and a fit-tested respirator.

Some diseases stack categories. Varicella is often airborne plus contact because skin lesions shed virus and droplet nuclei travel. If signs conflict or the isolation type is unclear, stop and ask the nurse or infection-control resource before entering. Do not just get the draw done.

Breaking the chain: actions a laboratory assistant actually performs

Match the task to the link you are breaking:

• Infectious agent: use the assigned hospital disinfectant with the required wet time on chairs, trays, and glucometers; never use a wipe that has already dried.
• Reservoir: contain specimens in leak-proof, biohazard-labeled containers; do not store food in the specimen refrigerator; treat used tourniquets and holders as contaminated until cleaned.
• Portal of exit: engage the safety device immediately after venipuncture; cover tubes before transport; do not centrifuge uncapped tubes.
• Mode of transmission: hand hygiene before and after each patient and after glove removal; never wear the same gloves from one patient to the next; clean shared equipment; follow isolation signage; leave your phone out of isolation rooms.
• Portal of entry: keep your own skin intact, cover cuts, never recap with two hands, use a face shield if splash is reasonably anticipated (popping a tube cap, pouring urine).
• Susceptible host: maintain immunity (hepatitis B vaccination is the bloodborne example developed in section 2.3), stay home when policy requires if you have a contagious illness, and use extra care on units with immunocompromised patients.

Engineered sharps, the Needlestick Safety and Prevention Act, and the written exposure control plan are Chapter 3. Here, two-handed recapping is simply a portal-of-entry failure you must not commit.

Hand hygiene

Hand hygiene is the highest-yield single break in the chain. Use alcohol-based hand rub when hands are not visibly soiled. Use soap and water when hands are visibly dirty, after using the restroom, and when caring for patients with known or suspected spore-forming or non-enveloped pathogens (C. difficile, norovirus) per CDC and facility policy. Cover all surfaces of the hands and allow alcohol to dry; do not wave wet hands dry and then glove. Gloves do not replace hand hygiene. Remove gloves, then clean hands, even if the gloves did not look dirty. Wearing gloves down the hall to the next room is a contact-transmission error.

PPE donning and doffing order

CDC donning (clean to dirty, before entry):

1. Gown
2. Mask or respirator
3. Goggles or face shield
4. Gloves (extended over the gown cuffs)

CDC doffing (most contaminated first; perform hand hygiene as directed between steps and always at the end):

1. Gloves
2. Goggles or face shield
3. Gown
4. Mask or respirator — last, and for airborne isolation after leaving the room with the door closed

A common alternative doffs gown and gloves together. Follow your facility's posted sequence, but on CMLA expect the classic CDC order: mask or respirator comes off last, and gloves go on last when donning. Touching the front of a mask or an N95 with contaminated gloves is a classic self-inoculation error (portal of entry through mucous membranes).

N95 versus surgical mask

Facial hair that breaks the seal, an expired fit test, or a damp crushed N95 means you do not have airborne protection. A surgical mask under an ill-fitting N95 does not fix a failed seal.

Scenario

You are sent to draw a basic metabolic panel on a patient whose door shows airborne isolation. You gather tubes in the hallway, perform hand hygiene, don gown, N95, eye protection, then gloves, enter, keep the door closed, complete the draw, bag the specimen, doff gloves, goggles, and gown in the room, exit, close the door, remove the N95, and perform hand hygiene again. That sequence breaks the mode-of-transmission and portal-of-entry links for you and the next patient.