Woods Lamp Examination Test: Purpose, Test Details & Results

The Woods lamp examination is a quick, non-invasive diagnostic test that uses ultraviolet (UV) light to help clinicians detect and evaluate a range of skin, hair, nail, and sometimes even eye and urine abnormalities. Although it's been a staple in dermatology for decades, its applications have broadened across multiple medical fields, providing valuable clues for diagnosis and management. In this article, we'll explore the core purposes behind the test, walk through what to expect during the examination, and break down how to interpret the results, including what happens next based on findings.

Purpose of Woods Lamp Examination Test

The Woods lamp test is much more than a simple flashlight inspection. It’s an essential clinical tool that leverages the unique properties of UV light to reveal changes in skin pigmentation, detect certain infectious agents, and even assist with the diagnosis of metabolic or toxic disorders. Understanding why and when a Woods lamp is used puts its value into perspective across dermatology, emergency medicine, and beyond.

Table 1: Core Purposes of Woods Lamp Examination

Understanding the Diagnostic Utility

Woods lamp examination is indispensable in dermatology for distinguishing between various types of pigmentation changes. For example, it can help determine whether excess melanin is located in the epidermis or dermis, which is crucial for diagnosing conditions like melasma or vitiligo. Under UV light, epidermal pigmentation stands out more sharply, whereas dermal pigment appears less distinct, guiding both diagnosis and treatment decisions 2 10.

Detecting Infections and Fungal Diseases

The test is frequently used to diagnose superficial fungal infections such as tinea capitis (scalp ringworm) and tinea versicolor, as well as bacterial infections like erythrasma. These pathogens can fluoresce under UV light, making them easier to spot during clinical exams, especially in resource-limited settings or when laboratory results are delayed 4 5 9.

Screening for Metabolic and Toxic Conditions

Outside of dermatology, the Woods lamp offers a rapid, point-of-care screening tool for certain metabolic or toxicological emergencies. For instance, it can detect porphyrins in the urine of patients with suspected acute intermittent porphyria, providing immediate clues before more definitive but slower lab tests are available 7. Similarly, it can help identify sodium fluorescein in urine after suspected antifreeze (ethylene glycol) poisoning—a critical first step in emergency care 1.

Eye and Forensic Applications

In ophthalmology, the Woods lamp can help detect corneal injuries when used with fluorescein dye, though its sensitivity is limited compared to a slit lamp examination 6. While sometimes touted for forensic use, such as detecting semen, studies have shown it’s unreliable for this purpose, with potential for false positives due to other substances fluorescing under UV light 8.

Woods Lamp Examination Test Details

If your clinician suggests a Woods lamp exam, you might wonder what’s involved. The procedure is straightforward, painless, and quick, but it does require a few special steps to ensure accurate results and keep everyone safe.

Table 2: Key Details of Woods Lamp Examination Procedure

Equipment and Setup

The Woods lamp itself is a compact, handheld device that emits long-wave ultraviolet (UV-A) light, typically around the 365 nm wavelength. This specific wavelength causes certain substances and organisms on or in the skin, hair, or nails to fluoresce—emit visible light—which can be seen by the examiner 3 5 6.

Patient and Room Preparation

To ensure the examination is effective and safe:

• The test should be performed in a completely dark room, so all regular lights and window shades are turned off 3 4.
• Skin, hair, or nails should be clean and free of makeup, lotions, or topical medications, as some substances can cause false fluorescence 10.
• For eye examinations, fluorescein dye may be applied to the eye before using the lamp to highlight corneal defects 6.

Safety Precautions

• Patients' eyes should be covered or protected to avoid potential injury from UV exposure, especially when the lamp is used near the face 3 6.
• The examiner should also avoid direct exposure to the eyes.

Step-By-Step Process

1. Explain the Test: The clinician explains the purpose and what the patient should expect.
2. Darken the Room: All lights are turned off.
3. Position the Patient: The area to be examined is exposed and appropriately cleaned.
4. Eye Protection: The patient’s eyes are shielded.
5. Use the Lamp: The Woods lamp is held a few centimeters from the skin, nail, or eye, and the area is carefully inspected for color changes or fluorescence.
6. Interpretation: Findings are documented, and the rest of the examination or diagnostic plan proceeds as needed.

Common Challenges and Tips

• Certain topical agents, soaps, or textile fibers may fluoresce and confound interpretation, so careful preparation is crucial 10.
• The lamp is best used as an adjunct to other diagnostic tools, not as a standalone test, especially for subtle or deep-seated disorders 9.

Woods Lamp Examination Test Results & Follow-Up

The Woods lamp exam offers immediate visual feedback, but interpretation depends on the condition being investigated and the context. Knowing what different fluorescence colors and patterns mean is key to accurate diagnosis and effective follow-up.

Table 3: Common Woods Lamp Findings and Their Diagnostic Value

Understanding Common Findings

Pigment Disorders

• Epidermal Hyperpigmentation: Appears enhanced with a clear border under UV light, helping differentiate superficial conditions (like melasma) from deeper ones 2 10.
• Vitiligo: Depigmented patches stand out as bright white or blue-white areas, often more clearly than under visible light 5.

Infectious Conditions

• Fungal Infections (Tinea capitis, versicolor):
  • May fluoresce with a distinctive yellow-green or blue-green hue, depending on the organism 4 9.
  • Erythrasma (caused by Corynebacterium minutissimum) typically glows coral-red 5.
  • However, not all fungal species fluoresce, so a negative result doesn't rule out infection 4 9.

Metabolic and Toxic Disorders

• Porphyria: Urine may show a pink or red fluorescence, which is a strong clue for acute intermittent porphyria, especially in emergency settings 7.
• Antifreeze (ethylene glycol) ingestion: Urine may fluoresce due to sodium fluorescein, aiding rapid diagnosis while awaiting lab confirmation 1.

Eye Findings

• Corneal Abrasions/Ulcers: When combined with fluorescein dye, defects in the corneal surface may fluoresce green. However, sensitivity is moderate, and slit lamp examination is preferred for definitive diagnosis 6.

Limitations and False Positives

• Not all conditions or organisms fluoresce; for example, many fungal and bacterial species are non-fluorescent 4 9.
• Certain substances (e.g., ointments, creams, textile fibers) may fluoresce and lead to misinterpretation if not properly accounted for 8 10.
• Forensic use, such as semen detection, is unreliable—semen does not consistently fluoresce under UV, and other substances can mimic its appearance 8.

What Happens Next?

• Positive Findings: Usually prompt further confirmatory testing, such as KOH mounts for fungi, laboratory assays for porphyrins, or more detailed eye exams.
• Negative or Inconclusive Results: Do not rule out disease. Further testing or clinical correlation is necessary, especially in high-suspicion cases 6.

Follow-Up Recommendations

• Interpretation should always be combined with clinical assessment and, when indicated, laboratory or histopathological studies 5 6 9.
• In suspected emergencies (e.g., toxic ingestions, acute porphyria), the Woods lamp can expedite triage, but confirmatory laboratory testing is essential 1 7.
• For chronic skin conditions, the Woods lamp can help guide treatment (e.g., determining depth of pigment, localizing infection for debridement) and monitor response 2 9.

Conclusion

The Woods lamp examination remains a valuable, user-friendly diagnostic tool across multiple medical specialties. By illuminating otherwise hidden features with UV light, it facilitates rapid diagnosis and management of pigment disorders, infections, and even some metabolic and toxicological emergencies.

Key takeaways:

• Versatile Tool: Used for pigment analysis, infection detection, metabolic/toxic screening, and more 1 2 4 5 7.
• Simple Procedure: Non-invasive, quick, and safe when performed correctly in a dark room with eye protection 3 5 6.
• Immediate Results: Offers real-time visual clues, but findings must be interpreted in context with clinical and laboratory data 4 6 9.
• Limitations Exist: Not all pathogens or conditions fluoresce; false positives and negatives can occur, so further testing is often needed 4 6 8 9.
• Guides Management: Helps clinicians triage emergencies, select appropriate therapies, and track treatment responses 1 2 7 9.

Whether you're a patient curious about what to expect or a clinician seeking to refine your technique, understanding the purpose, process, and interpretation of the Woods lamp test ensures it remains a bright spot in clinical diagnostics.