13.4 Stains, KOH, Wet Preps, Reagents, and Controls

AMT III.8.B.4 is assemble reagents, standards, and controls for microbiological procedures. III.8.B.5 is prepare and stain slides for further analyses. The competency checklist says the same in two lines: process microbiologic specimens aseptically, and prepare and stain Gram stain slides for further analysis. “Further analysis” is the scope word. A CMLA makes a readable slide. A medical laboratory technician (MLT), medical laboratory scientist (MLS), or other qualified testing personnel typically reads Gram morphology, KOH elements, AFB smears, and C&S. You do not independently sign those out, and you do not release AST from a stain rack.

Quick Answer: Line up in-date stains, KOH, saline, controls, and media. Gram stain: crystal violet → iodine → decolorize → safranin. You prepare; qualified staff read. KOH clears keratin for fungi. Wet prep is saline, unstained. AFB may be processed under SOP; interpretation is not independent CMLA work. QC known organisms and media lots. Expired reagent is a stop.

Assemble reagents, standards, and controls first

Before any smear hits a slide, gather what the SOP names:

• Stain set: crystal violet, Gram’s iodine, decolorizer (acetone-alcohol), safranin (or the kit’s counterstain)
• Immersion oil for the reader (oil is for the 100× objective, not for you to “preview” diagnosis)
• KOH reagent (commonly 10%) and saline for wet mounts
• AFB reagents only if the SOP assigns you that processing step
• Labeled slides, a marking method that survives staining, and a slide rack
• Controls: at minimum, known gram-positive and gram-negative organisms for Gram stain per SOP frequency; media sterility and growth controls as the laboratory’s QC plan requires
• Standards in this outline sense are the in-control organisms, the stain-timing chart, and the manufacturer’s expected reactions—not a chemistry calibrator you invent

Check expiration, open-date dating, storage temperature, and appearance. Crystal violet that has precipitated, iodine that has lightened, or KOH that crystallized is not “close enough.” Chapter 4 already taught out-of-range temperature logs; stain bottles sitting on a sunny windowsill are the same problem. Do not top off a half-empty stain jar with a different lot.

Gram stain steps at assistant level—and who reads the slide

Make a thin, even smear from the specimen or from a colony if SOP has you prepare colony smears. Air-dry. Heat-fix or methanol-fix as the SOP says—overheating distorts cells. Then stain:

Rinse between steps as trained; drain, do not hose the smear off the slide. Timing is SOP-specific; count it, do not chat through decolorizer. After the slide is dry, it goes to the person credentialed to interpret gram-positive cocci, gram-negative rods, yeasts, and host cells.

Who reads: not independent CMLA sign-out. You may document that the slide was prepared and that stain QC passed. You do not phone a provider with “it’s GNR, start antibiotics” from your glance at 40×. Mixed flora versus a single morphology, sputum quality (epithelial cells versus neutrophils), and whether a blood-culture bottle is a true pathogen are interpretive.

KOH and wet prep

KOH (potassium hydroxide) dissolves keratin and debris in skin, nail, and mucous-membrane specimens so hyphae and yeast can be seen. Place the sample on a slide, add KOH, coverslip, and wait the SOP time (warming may be allowed; boiling the patient off the slide is not). Fungal elements survive; many human cells do not. A CMLA prepares the mount. Identification of dermatophytes versus artifact is typically not assistant independent interpretation.

A wet prep (wet mount) uses saline, not KOH and not Gram reagents. It is for motility and structures that stains would kill or distort: trichomonads, clue cells, yeast, some stool parasites depending on SOP. Do not air-dry a wet prep and then Gram-stain it as if that were the same test.

Some vaginal wet mounts are provider-performed microscopy (PPM). Regulations restrict PPM to providers (Chapter 16). Preparing the slide for a provider or for the laboratory’s qualified reader is in scope when SOP says so. Independently reporting trichomonas from a PPM wet mount because “I saw it wiggle” is not a CMLA loophole.

AFB smear: process versus interpret

An AFB smear looks for acid-fast bacilli. Classic carbolfuchsin methods (Ziehl–Neelsen, Kinyoun) and fluorescent auramine stains are used in mycobacteriology. Digestion, decontamination, and staining of sputum for AFB are SOP-controlled and often restricted because of aerosol risk (BSL practices, BSC). A CMLA may stain or process if the facility has trained and competency-checked that task. Interpretation—how many AFB per field, whether the smear is positive, whether it supports a tuberculosis diagnosis—is not independent CMLA sign-out. Do not treat an AFB reagent rack as a waived cassette, and do not use Gram-stain reagents as a substitute “quick AFB.”

QC of stains and media

Stains lie when QC is skipped.

Media QC is why you assemble plates and do not grab the oldest cracked blood agar from an unlabeled drawer. Hemolysis patterns and A&P disks only mean something on media that still supports growth. Incubator temperature logs from Chapter 4 still apply: a 40°C incubator is not “faster culture.”

If stain QC fails, patient smears prepared in that run are invalid until the problem is corrected. That is the same stop-the-line logic as failed waived-kit internal control (Chapter 5), not a reason to “read around” the control slide.

Scenario

You stain a wound smear and a control slide. The known gram-positive control is pink, and the known gram-negative control is also pink. Do not send the patient slide with a note “probably over-decolorized but looks like rods.” Repeat with fresh, timed decolorizer or new reagents until controls stain correctly, then restain the patient smear for the qualified reader. Expired safranin does not get a patient result.

Exam traps

• Safranin first, or skipping iodine.
• CMLA independently signing out Gram, AFB, KOH identification, C&S, or AST.
• Treating KOH as a tuberculosis stain or a chemistry electrolyte.
• Calling a wet prep a Gram stain.
• Using expired plates or stains because “colonies still appeared.”
• Performing PPM wet-mount diagnosis as an assistant.

Prepare the slide as if someone else’s license depends on it—because it does. Then leave the interpretation in that license’s scope.