KOH Test- Principle, Media, Procedure, Results, Uses
KOH test is a simple and rapid test that can be used to visualize fungal structures in clinical samples. It is based on the principle that potassium hydroxide (KOH) can dissolve the cellular components of human and bacterial cells, but not the fungal cells. This makes the fungal cells stand out against a clear background and allows their identification based on their morphology and arrangement.
KOH test is useful for the presumptive identification of dermatophytes, which are fungi that cause infections of the skin, hair and nails. It can also help in the preliminary diagnosis of other mycoses, which are diseases caused by fungi. KOH test can be performed on various types of clinical specimens, such as skin scrapings, nail clippings, hair, pus, sputum, cerebrospinal fluid (CSF), tissue samples, urine, mucous membrane swabs, bronchial and alveolar washings, etc.
KOH test is easy to perform and does not require complex or expensive equipment or reagents. It can provide results within minutes and help in selecting and initiating appropriate antifungal therapy. However, KOH test has some limitations as well. It cannot provide definitive identification of fungal species or differentiate between viable and non-viable fungal cells. It also requires a skilled person to interpret the results and may not work well with some complex tissue materials that are resistant to KOH digestion.
In this article, we will discuss the objectives, principle, requirements, procedure, results, precautions, applications, advantages and disadvantages of KOH test in detail. We will also compare some modified KOH mount methods that can improve the contrast and staining of fungal cells.
The KOH test is a simple and quick test that can be performed in any laboratory setting to visualize fungi and their structures in a clinical specimen. The main objectives of the KOH test are:
- To make a presumptive identification of dermatophytes, which are fungi that cause infections of the skin, hair and nails. Dermatophytes can be classified into three genera: Microsporum, Trichophyton and Epidermophyton. The KOH test can help to differentiate them based on their morphology and location in the hair shaft (endothrix or ectothrix).
- To make a preliminary diagnosis of mycoses, which are diseases caused by fungi. Some common mycoses include candidiasis, aspergillosis, cryptococcosis, histoplasmosis and blastomycosis. The KOH test can help to detect the presence of yeast cells, pseudohyphae, hyphae, spores and other fungal elements in various specimens such as skin scrapings, nail clippings, sputum, pus, urine and tissue samples.
- To confirm the results of other diagnostic tests such as culture, serology and molecular methods. The KOH test can provide a rapid confirmation of fungal growth in culture media or a positive serological or molecular test. The KOH test can also help to rule out false-positive results due to contamination or cross-reaction.
The principle of KOH test is based on the fact that potassium hydroxide (KOH) can dissolve the proteinaceous and cellular materials in the clinical specimens, leaving behind the fungal elements intact and visible. KOH acts as a clearing reagent that breaks down the keratin, adhesives, and other alkali-sensitive substances that hold the keratinized cells together. This digestion results in the transparency of the specimen and the release of the bound fungal components. The fungal components, however, are alkali resistant so they remain intact and unaffected by KOH. These allow clear visualization of the microscopic morphology of the fungi and fungal elements in the sample.
The KOH test can also be modified by adding other reagents to enhance the contrast and staining of the fungal elements. For example, calcofluor white is a fluorescent dye that binds to the cellulose and chitin in the fungal cell wall and emits blue light when exposed to ultraviolet light. This makes the fungi more visible and distinguishable from the background. Another example is dimethyl sulfoxide (DMSO), which is a solvent that enhances the penetration and clearing action of KOH. This makes the digestion process faster and more complete.
The KOH test is a simple, rapid, and inexpensive method to visualize fungi and their structures in a clinical specimen. It can also provide presumptive identification of some fungal genera based on their characteristic morphology and arrangement. However, it cannot provide definitive identification of fungal species or differentiate between pathogenic and non-pathogenic fungi. Therefore, it is usually used as a preliminary test to confirm the presence of fungi and guide further diagnostic tests such as culture, biochemical tests, or molecular methods.
To perform the KOH test, you will need the following:
- 10% to 30% KOH solution: This is the main reagent that dissolves the tissue materials and clears the specimen. The concentration of KOH depends on the type of specimen. You can prepare the KOH solution by adding the appropriate amount of KOH pellets to distilled water and stirring well. For example, to make 100 mL of 20% KOH solution, you will need 20 grams of KOH pellets and 80 mL of distilled water.
- Optional: Calcofluor white or DMSO: These are additives that can enhance the visualization of fungal elements by staining them or improving their clearing. You can add 0.01% calcofluor white or 40% DMSO to the KOH solution according to your preference. For example, to make 100 mL of 20% KOH with 0.01% calcofluor white, you will need 0.1 grams of calcofluor white powder and 80 mL of 40% DMSO solution.
- PPE and other general laboratory materials: You should wear gloves, goggles, lab coat and other protective equipment to avoid contact with the KOH solution and the specimens. You will also need sterile glass slides, cover slips, forceps, scalpel, scissors, filter paper, petri plate, beaker, stirrer, etc.
- Microscope: You will need a compound light microscope with at least 400X magnification to observe the fungal structures. You may also use a fluorescence microscope if you are using calcofluor white as a stain.
You can use any type of clinical specimen that may contain fungi or fungal elements. Some common examples are:
- Skin scrapings: These are obtained by scraping the affected area of the skin with a scalpel or a glass slide. You should collect the material from the edge of the lesion where the fungi are most likely to be present.
- Nail clippings: These are obtained by cutting the infected part of the nail with scissors or a nail clipper. You should collect the material from underneath the nail plate where the fungi are most likely to be present.
- Hair: These are obtained by plucking or cutting the infected hair with forceps or scissors. You should collect the material from the root or shaft of the hair where the fungi are most likely to be present.
- Pus: This is obtained by aspirating or draining the infected site with a syringe or a needle. You should collect the material from the center of the lesion where the fungi are most likely to be present.
- Sputum: This is obtained by coughing up or expectorating the respiratory secretions from the lungs or bronchi. You should collect the material from the first morning sputum where the fungi are most likely to be present.
- CSF: This is obtained by performing a lumbar puncture or a spinal tap on the lower back. You should collect the material from the clear fluid that surrounds the brain and spinal cord where the fungi are most likely to be present.
- Tissue (biopsy) sample: This is obtained by performing a surgical procedure or a needle biopsy on the affected organ or tissue. You should collect the material from the site of infection where the fungi are most likely to be present.
- Urine: This is obtained by collecting a midstream urine sample in a sterile container. You should collect the material from the first morning urine where the fungi are most likely to be present.
- Mucous membrane swabs: These are obtained by swabbing the affected area of the mouth, throat, nose, vagina, etc. with a sterile cotton swab. You should collect the material from the site of infection where the fungi are most likely to be present.
- Bronchial and alveolar washings: These are obtained by performing a bronchoscopy or a lavage on the lower respiratory tract. You should collect the material from the fluid that washes the bronchi and alveoli where the fungi are most likely to be present.
There are two main methods of performing the KOH test: the KOH mount method and the modified KOH mount methods. The KOH mount method is the basic technique that uses only KOH solution as a reagent, while the modified KOH mount methods use additional substances to enhance the visualization of fungal elements.
KOH Mount Method
The KOH mount method involves the following steps:
- In a clean and sterile glass slide, place a drop of KOH solution. The concentration of KOH depends on the type of specimen. For sputum, pus, CSF, urine, and swabs, 10% KOH is sufficient. For skin scrapings, some swabs with lots of tissue debris, tissue samples, and thick pus, 20% KOH is recommended. For nail clippings and hair, 30% KOH is required.
- Transfer a small portion of the specimen over the KOH drop and place a cover slip on top.
- Incubate the specimen-KOH mixture at room temperature for 5 to 30 minutes (time varies according to specimen) for clearing the sample and digesting cellular debris. Alternatively, you can heat the slide over a flame or a heating block or incubate at 30°C to accelerate the digestion process. However, do not let the KOH boil or dry out. To avoid this, place the slide over moistened filter paper on a petri plate.
- Examine under a compound light microscope; first at low power (100 X magnification), then shift to high power (400 X magnification). No need to magnify 1000 times.
- Observe the fungal morphology, arrangement of fungal cells/hyphae, morphology and arrangement of fungal spores, and in the case of hair specimen, examine the location of fungi in hairs.
Modified KOH Mount Methods
The modified KOH mount methods are variations of the basic technique that use different reagents to improve the contrast and coloring of fungal structures. Some of the commonly used modified methods are:
In this method, calcofluor white (a fluorescent blue dye that binds with cellulose and chitin of fungal components) is added to the KOH solution to stain the sample. The procedure is similar to the basic method, except that the reagent is changed from 10 to 30% KOH to 10 to 30% KOH with 0.01% calcofluor white. The slide is then examined under a fluorescence microscope.
In this method, dimethyl sulfoxide (DMSO) is added to the KOH solution to enhance the clearing of the specimen during visualization. The procedure is similar to the basic method, except that the reagent is changed from 10 to 30% KOH to 20% KOH in 40% DMSO in distilled water.
After performing the KOH test, examine the slide under a microscope and look for the presence and morphology of fungal elements. Depending on the type of specimen and fungus, you may see different structures and features. Here are some examples of what you may observe:
- Yeast cells: These are round or oval cells that may be single or in clusters. They may also show budding, which is a form of asexual reproduction where a small outgrowth emerges from the parent cell. Yeast cells are typically seen in specimens from mucous membranes, such as oral or vaginal swabs. Candida albicans is a common yeast that causes candidiasis or thrush.
- Pseudohyphae: These are elongated cells that resemble hyphae (fungal filaments) but are actually chains of yeast cells. They may have constrictions or septa at regular intervals. Pseudohyphae are also seen in specimens from mucous membranes and are indicative of Candida albicans infection.
- Fungal mycelium: This is the network of hyphae that forms the body of most fungi. Hyphae are long, thin, branching filaments that may be septate (divided by cross-walls) or aseptate (without cross-walls). Fungal mycelium is usually seen in specimens from skin, hair, nails, or tissues. Depending on the type of fungus, the mycelium may have different characteristics, such as color, texture, width, branching pattern, etc.
- Spores: These are reproductive structures that are produced by fungi either sexually or asexually. Spores may have different shapes, sizes, colors, and arrangements depending on the fungal species. Some common types of spores are conidia (asexual spores produced at the tips or sides of hyphae), sporangiospores (asexual spores produced inside a sac-like structure called sporangium), and chlamydospores (thick-walled spores that serve as survival structures). Spores are usually seen in specimens from skin, hair, nails, or tissues.
- Hair lesions: These are abnormalities in the structure or appearance of hair caused by fungal infection. Hair lesions may be classified as endothrix (when the fungus grows inside the hair shaft) or ectothrix (when the fungus grows outside the hair shaft). Hair lesions are seen in specimens from scalp or beard hair. Microsporum and Trichophyton are common genera of fungi that cause hair lesions.
To interpret the result of KOH test, compare the observed fungal elements with the expected morphology and arrangement of different fungal species. For example:
- If you see round yeast cells with pseudohyphae forming a "spaghetti and meatballs" appearance, you can presume that it is Candida albicans infection.
- If you see ribbon-like aseptate hyphae with right-angle branching, you can presume that it is Mucor infection.
- If you see septate hyphae with dichotomous branching at 45 degrees and conidia in chains or clusters, you can presume that it is Aspergillus infection.
- If you see septate hyphae with club-shaped macroconidia and microconidia along the sides, you can presume that it is Microsporum infection.
- If you see septate hyphae with pencil-shaped macroconidia and microconidia at the tips, you can presume that it is Trichophyton infection.
However, keep in mind that KOH test is only a presumptive identification technique and cannot provide definitive diagnosis of fungal species. Therefore, it is advisable to confirm the result with other methods such as culture, biochemical tests, molecular tests, etc.
- Wear appropriate personal protective equipment (PPE) such as gloves, goggles, and lab coat to avoid contact with KOH solution, which is corrosive and can cause skin irritation and eye damage.
- Use sterile and clean glass slides and cover slips to avoid contamination of the specimen and the reagent.
- Use a fresh KOH solution for each test and discard the used solution in a proper waste container. Do not reuse or store the KOH solution for a long time as it may lose its potency and clarity.
- Use the correct concentration of KOH solution according to the type of specimen. For example, use 10% KOH for sputum, pus, CSF, urine, and swabs; 20% KOH for skin scrapings, thick swabs, thick pus, and tissue samples; and 30% KOH for hair and nail clippings.
- Transfer a small amount of specimen to the KOH drop on the slide and spread it evenly with a sterile needle or spatula. Do not use too much or too little specimen as it may affect the digestion and visualization of fungal elements.
- Incubate the slide at room temperature for the appropriate time depending on the specimen. Do not over-incubate or under-incubate the slide as it may result in incomplete digestion or over-digestion of the specimen. If heating is required, do not let the KOH solution boil or dry out as it may damage the fungal elements or cause false-negative results. To prevent evaporation, place the slide over moistened filter paper on a petri plate during incubation.
- Examine the slide under a compound light microscope using low power (100X) and high power (400X) objectives. Do not use oil immersion (1000X) objective as it may interfere with the clarity of the KOH solution. Adjust the light intensity and focus carefully to obtain a clear view of the fungal elements. Look for yeast cells, pseudohyphae, hyphae, spores, and other characteristic features of fungi in different fields of view.
- Compare the observed morphology with reference images or descriptions of common fungi to make a presumptive identification. If possible, confirm the identification by performing other tests such as culture, biochemical tests, or molecular methods.
- Record and report the results accurately and promptly. Include the type of specimen, concentration of KOH solution, incubation time, microscopic findings, and presumptive identification if any. For example: "Skin scraping; 20% KOH; 20 minutes; septate hyphae with conidia suggestive of Trichophyton spp."
- Clean and disinfect the glass slides, cover slips, needles, spatulas, microscope, and other equipment after use. Dispose of the used PPE and specimen in a biohazard bag according to the laboratory safety guidelines.
The KOH test is a simple and quick method to visualize fungi and their structures in various clinical specimens. It has several applications in the diagnosis and management of fungal infections, such as:
- Visualization of fungal morphology: The KOH test allows the observation of the microscopic features of fungi, such as yeast cells, pseudohyphae, hyphae, spores, and reproductive structures. These features can help to differentiate between different groups of fungi, such as yeasts, molds, and dimorphic fungi. The morphology can also provide clues to the identification of some specific fungal genera or species, such as Candida, Aspergillus, Dermatophytes, etc.
- Confirmation of fungal species in a clinical sample: The KOH test can be used to confirm the presence or absence of fungi in a clinical sample that has been cultured or stained by other methods. For example, the KOH test can be used to confirm the growth of fungi on Sabouraud dextrose agar (SDA) or the staining of fungi by Gram stain or periodic acid-Schiff (PAS) stain. The KOH test can also be used to rule out contamination or overgrowth of bacteria or other microorganisms in a fungal culture or stain.
- Preliminary diagnosis of mycoses to select and start appropriate antimicrobial therapy: The KOH test can provide a rapid and preliminary diagnosis of some common fungal infections, such as dermatophytosis (ringworm), candidiasis (thrush), pityriasis versicolor (tinea versicolor), and aspergillosis. Based on the KOH test results, the clinician can select and start an appropriate antifungal therapy for the patient without waiting for the final identification and susceptibility testing of the fungus. This can reduce the morbidity and mortality associated with fungal infections, especially in immunocompromised patients.
- Identification of pityriasis versicolor: The KOH test is a reliable and specific method to identify pityriasis versicolor, a superficial skin infection caused by Malassezia spp. The KOH test reveals the characteristic "spaghetti and meatballs" appearance of the yeast cells and short hyphae of Malassezia spp. in the skin scales. The KOH test can also differentiate pityriasis versicolor from other skin conditions that may have similar clinical manifestations, such as vitiligo, seborrheic dermatitis, or psoriasis.
- Differentiation of yeasts and filamentous fungi: The KOH test can help to distinguish between yeasts and filamentous fungi based on their morphology and arrangement. Yeasts are unicellular fungi that appear as round or oval cells that may form pseudohyphae (elongated cells with constrictions). Filamentous fungi are multicellular fungi that form hyphae (long and branching filaments) that may have septa (cross-walls) or be aseptate (without cross-walls). The differentiation of yeasts and filamentous fungi is important for the selection of appropriate culture media and identification methods.
The KOH test is a simple and quick method to visualize fungal structures in clinical samples. It has some advantages and disadvantages/limitations that should be considered before performing or interpreting the test.
Advantages of KOH Test
- It is easy to perform and provides rapid results. The test can be done within minutes to hours depending on the specimen type and concentration of KOH used.
- It does not require complex and expensive materials/reagents and skills. The test can be performed with a basic microscope, glass slides, cover slips, and KOH solution. The KOH solution can be easily prepared in the laboratory or purchased from suppliers.
- It is safe to perform. The KOH solution is not toxic or hazardous to handle. The specimens are not infectious after being treated with KOH.
- It is suitable for all types of clinical specimens. The test can be applied to skin scrapings, nail clippings, hair, pus, sputum, CSF, tissue samples, urine, mucous membrane swabs, bronchial and alveolar washings, etc.
- It can provide useful information for the preliminary diagnosis of mycoses and the selection of appropriate antifungal therapy. The test can reveal the presence or absence of fungi and their morphology in the specimen. It can also help to differentiate between yeasts and filamentous fungi, and between some common fungal genera such as Candida, Aspergillus, Dermatophytes, etc.
Disadvantages/Limitations of KOH Test
- It is a presumptive identification technique only; therefore, it cannot exactly identify the fungal species. The test cannot distinguish between different species of the same genus or between closely related genera. For example, it cannot differentiate between Candida albicans and Candida glabrata, or between Aspergillus fumigatus and Aspergillus flavus. The test also cannot detect mixed infections by different fungi or co-infections by bacteria or parasites.
- It does not provide color and contrast to the fungal structures. The KOH solution clears the background tissue but does not stain the fungal elements. This makes it difficult to see the fungal structures against the transparent background. Some fungal structures may be too small or faint to be visible under the microscope. The use of modified KOH methods with calcofluor white or DMSO can improve the contrast and visibility of the fungal structures but may also increase the cost and complexity of the test.
- It requires a skilled person to interpret the results. The test relies on the observation of the microscopic morphology of the fungi and their elements in the specimen. This requires a good knowledge of fungal taxonomy and morphology as well as experience in microscopy. The interpretation may also be subjective and prone to errors due to variations in specimen quality, KOH concentration, digestion time, magnification, illumination, etc.
- Some complex tissue materials may require a long time to dissolve or may not dissolve at all and cause problems during the visualization of fungi. For example, nail clippings and hair may take up to 30 minutes or longer to digest with 30% KOH solution. Some tissue samples may contain keratinized cells or other substances that are resistant to KOH digestion and may obscure the fungal structures. In such cases, alternative methods such as histopathology or culture may be needed to confirm the presence of fungi.
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