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Diagnostic Tests – Fine-Needle Aspiration (FNA) of the Thyroid
1. What is this test?
Fine-needle aspiration (FNA) of the thyroid is a minimally invasive procedure in which a thin needle is used to obtain a sample of thyroid cells for cytologic examination. It is the initial diagnostic test of choice for most solitary thyroid nodules, as it helps determine whether a nodule is benign or malignant.
2. When is this the most accurate test?
FNA is the most appropriate and accurate next step when a patient has:
In exam questions, if a euthyroid patient with a thyroid nodule is described and the question asks for the next step in diagnosis, the correct answer is typically fine-needle aspiration.
3. When is this the wrong answer?
FNA is not the correct initial test in patients with a solitary thyroid nodule who are thyrotoxic (suppressed TSH, elevated T3/T4). In this scenario, a thyroid scintigraphy (radioiodine uptake scan) should be performed first to determine whether the nodule is:
1. What is this test?
Fine-needle aspiration (FNA) of the thyroid is a minimally invasive procedure in which a thin needle is used to obtain a sample of thyroid cells for cytologic examination. It is the initial diagnostic test of choice for most solitary thyroid nodules, as it helps determine whether a nodule is benign or malignant.
2. When is this the most accurate test?
FNA is the most appropriate and accurate next step when a patient has:
- A palpable solitary thyroid nodule
- Normal thyroid function tests (euthyroid state)
In exam questions, if a euthyroid patient with a thyroid nodule is described and the question asks for the next step in diagnosis, the correct answer is typically fine-needle aspiration.
3. When is this the wrong answer?
FNA is not the correct initial test in patients with a solitary thyroid nodule who are thyrotoxic (suppressed TSH, elevated T3/T4). In this scenario, a thyroid scintigraphy (radioiodine uptake scan) should be performed first to determine whether the nodule is:
- Hyperfunctioning (“hot”) — almost never malignant
- Hypofunctioning (“cold”) — higher risk of malignancy and may then require FNA
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Diagnostic Tests – Diascopy
1. For which clinical condition(s) is this test commonly used?
Diascopy is a simple bedside test used in dermatology for the initial evaluation of erythematous skin lesions. It helps determine whether skin redness is due to vasodilation or extravasation of red blood cells from damaged blood vessels. The test is performed by pressing a clear glass or plastic slide against the lesion.
2. What do you look for?
During diascopy, you observe whether the lesion blanches (turns white) when pressure is applied:
This distinction is clinically important, as non-blanching lesions often suggest more serious underlying pathology requiring further investigation.
1. For which clinical condition(s) is this test commonly used?
Diascopy is a simple bedside test used in dermatology for the initial evaluation of erythematous skin lesions. It helps determine whether skin redness is due to vasodilation or extravasation of red blood cells from damaged blood vessels. The test is performed by pressing a clear glass or plastic slide against the lesion.
2. What do you look for?
During diascopy, you observe whether the lesion blanches (turns white) when pressure is applied:
- Blanching lesion
→ Indicates vasodilation
→ Common in viral exanthems, drug reactions, and insect bites - Non-blanching lesion
→ Indicates extravasation of erythrocytes due to vascular damage
→ Seen in vasculitic and purpuric conditions
This distinction is clinically important, as non-blanching lesions often suggest more serious underlying pathology requiring further investigation.
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Diagnostic Tests – Direct Immunofluorescence (DIF) of Skin Biopsy
1. For which clinical condition(s) is this the most accurate test?
Direct immunofluorescence (DIF) is the most accurate diagnostic test for immunologic blistering skin disorders, particularly:
It is also used for other immune-mediated dermatoses where antibody or complement deposition patterns are diagnostic.
2. When is this test the best initial step?
DIF is the best initial diagnostic step when a patient presents with:
The test is performed on a skin biopsy taken from perilesional (normal-appearing) skin, not from the blister itself.
DIF evaluates the presence and pattern of deposition of:
Management depends on the diagnosis confirmed by DIF:
1. For which clinical condition(s) is this the most accurate test?
Direct immunofluorescence (DIF) is the most accurate diagnostic test for immunologic blistering skin disorders, particularly:
- Pemphigus vulgaris
- Bullous pemphigoid
It is also used for other immune-mediated dermatoses where antibody or complement deposition patterns are diagnostic.
2. When is this test the best initial step?
DIF is the best initial diagnostic step when a patient presents with:
- Blistering skin disease
- Suspicion of an autoimmune etiology
- Findings such as fragile bullae, mucosal involvement, or tense blisters
The test is performed on a skin biopsy taken from perilesional (normal-appearing) skin, not from the blister itself.
DIF evaluates the presence and pattern of deposition of:
- IgG
- IgM
- IgA
- C3
- Fibrinogen
Management depends on the diagnosis confirmed by DIF:
- Pemphigus vulgaris
- DIF finding: IgG and C3 between epidermal cells (fish-net pattern)
- Treatment: Systemic corticosteroids ± immunosuppressants
- Bullous pemphigoid
- DIF finding: IgG and C3 at the dermoepidermal junction (linear pattern)
- Treatment: Topical or systemic corticosteroids, depending on severity
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Diagnostic Tests – Skin Scraping and Mineral Oil Mount
1. What is this test?
A skin scraping with mineral oil mount is a bedside diagnostic test used to identify scabies. Mineral oil is applied to the suspected lesion to enhance visualization of the burrow. The skin is then scraped and examined under a microscope. The mineral oil preserves skin cells, mite feces (scybala), and allows mites to remain intact and motile, improving diagnostic yield.
2. When is this the answer?
This test is indicated when a patient presents with:
In exam questions, the combination of pruritus + burrows + web-space involvement should immediately point to scabies, making skin scraping with mineral oil the correct diagnostic choice.
3. What is an adjuvant test often used in the same scenario?
A commonly used adjunct test is a KOH wet mount prepared from the same skin scrapings. The slide is gently heated to dissolve keratin and break up squamous cells, helping reveal mites that may be hidden beneath debris.
1. What is this test?
A skin scraping with mineral oil mount is a bedside diagnostic test used to identify scabies. Mineral oil is applied to the suspected lesion to enhance visualization of the burrow. The skin is then scraped and examined under a microscope. The mineral oil preserves skin cells, mite feces (scybala), and allows mites to remain intact and motile, improving diagnostic yield.
2. When is this the answer?
This test is indicated when a patient presents with:
- Intensely pruritic rash, often worse at night
- Small linear burrows, especially in the web spaces of the hands and feet
- Possible involvement of genitalia or breasts
- A history suggesting close personal or sexual contact
In exam questions, the combination of pruritus + burrows + web-space involvement should immediately point to scabies, making skin scraping with mineral oil the correct diagnostic choice.
3. What is an adjuvant test often used in the same scenario?
A commonly used adjunct test is a KOH wet mount prepared from the same skin scrapings. The slide is gently heated to dissolve keratin and break up squamous cells, helping reveal mites that may be hidden beneath debris.
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Diagnostic Tests – Wood’s Light Examination
1. What is this?
A Wood’s light examination is a noninvasive dermatologic test that uses ultraviolet (UV) light (~365 nm) to examine the skin, hair, and nails in a dark room. Certain organisms and skin conditions fluoresce under UV light, helping clinicians narrow the differential diagnosis of infectious and pigmentary disorders.
2. What are some organisms or conditions that fluoresce under Wood’s light?
Characteristic fluorescence patterns include
1. What is this?
A Wood’s light examination is a noninvasive dermatologic test that uses ultraviolet (UV) light (~365 nm) to examine the skin, hair, and nails in a dark room. Certain organisms and skin conditions fluoresce under UV light, helping clinicians narrow the differential diagnosis of infectious and pigmentary disorders.
2. What are some organisms or conditions that fluoresce under Wood’s light?
Characteristic fluorescence patterns include
- Erythrasma (Corynebacterium minutissimum)
→ Coral-red fluorescence - Tinea capitis (some Microsporum species)
→ Blue-green fluorescence - Pseudomonas aeruginosa
→ Green fluorescence - Vitiligo
→ Bright blue-white accentuation - Tinea versicolor (Malassezia)
→ Yellowish or coppery fluorescence (may be subtle or absent)
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Diagnostic Tests – Arthrocentesis
1. What is this?
Arthrocentesis is a procedure in which synovial fluid is aspirated from a joint using a needle for laboratory and microscopic analysis. It is the most accurate immediate test to distinguish between:
2. When do you answer arthrocentesis?
You should answer arthrocentesis when a patient presents with:
This is especially critical when infection must be ruled out. In cases of suspected gout, serum uric acid may be elevated, but arthrocentesis is still required to confirm the diagnosis and exclude infection.
Exam pearl:
Any new, hot, swollen joint = arthrocentesis first
3. What findings are evident in the two synovial fluid samples?
Microscopic examination under polarized light reveals characteristic crystals:
1. What is this?
Arthrocentesis is a procedure in which synovial fluid is aspirated from a joint using a needle for laboratory and microscopic analysis. It is the most accurate immediate test to distinguish between:
- Normal joint fluid
- Inflammatory arthritis
- Septic (infectious) arthritis
- 0–2,000 cells/µL → Normal
- 2,000–20,000 cells/µL → Inflammatory (e.g., gout, rheumatoid arthritis)
- 20,000–50,000 cells/µL → Indeterminate
- >50,000 cells/µL → Septic arthritis
2. When do you answer arthrocentesis?
You should answer arthrocentesis when a patient presents with:
- Acute onset of a swollen joint
- Warmth, pain, and tenderness
- Joint effusion with decreased range of motion
This is especially critical when infection must be ruled out. In cases of suspected gout, serum uric acid may be elevated, but arthrocentesis is still required to confirm the diagnosis and exclude infection.
Exam pearl:
Any new, hot, swollen joint = arthrocentesis first
3. What findings are evident in the two synovial fluid samples?
Microscopic examination under polarized light reveals characteristic crystals:
- Gout
- Negatively birefringent
- Needle-shaped crystals
- Composed of monosodium urate
- Pseudogout
- Positively birefringent
- Rhomboid-shaped crystals
- Composed of calcium pyrophosphate
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Diagnostic Tests – Patch Testing
1. For which clinical scenario(s) is this test indicated?
Patch testing is indicated in patients with suspected allergic contact dermatitis when:
The rash is chronic or recurrent
2. What important distinction must be made before utilizing this test?
Before performing patch testing, it is critical to distinguish between:
Patch testing is NOT useful for irritant contact dermatitis, because irritant reactions do not involve an immune response.
Patch testing should only be used when allergic contact dermatitis is suspected.
1. For which clinical scenario(s) is this test indicated?
Patch testing is indicated in patients with suspected allergic contact dermatitis when:
The rash is chronic or recurrent
- The suspected offending agent has already been removed
- Topical therapy (e.g., corticosteroids, emollients) has failed
- The clinician needs to identify the specific allergen responsible for the dermatitis
2. What important distinction must be made before utilizing this test?
Before performing patch testing, it is critical to distinguish between:
- Allergic contact dermatitis (immune-mediated, type IV hypersensitivity)
- Irritant contact dermatitis (non-immunologic, due to direct chemical or physical damage
Patch testing is NOT useful for irritant contact dermatitis, because irritant reactions do not involve an immune response.
Patch testing should only be used when allergic contact dermatitis is suspected.
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Diagnostic Tests — Adrenal & Cortisol Disorders
ACTH (Cosyntropin) Stimulation Test
What is this test?
- Evaluates adrenal cortex function
- Best initial diagnostic test for adrenal insufficiency
- Procedure:
- Measure baseline serum cortisol
- Administer cosyntropin (synthetic ACTH)
- Recheck cortisol at 30–60 minute
- Cortisol rises → adrenal glands functioning
- Cortisol does NOT rise → primary adrenal insufficiency (Addison disease)
When do you answer ACTH (Cosyntropin) Stimulation Test?
Chronic presentation (Addison disease):
- Fatigue, weakness
- Weight loss
- Hypotension
- Skin hyperpigmentation
- Hyponatremia
- Hyperkalemia
- Mild metabolic acidosis
- Salt craving (e.g., pickle juice)
→ Answer: ACTH stimulation test
Acute / ICU presentation:
- Unexplained hypotension
- Refractory to fluids or pressors
- History of chronic steroid use, autoimmune disease, or severe illness
→ Answer: ACTH stimulation test
Rapid Test Choice Guide
- Fatigue + hypotension + hyperpigmentation
- Hyperkalemia
- Hyponatremia
- Salt craving
→ ACTH (cosyntropin) stimulation test
- ICU patient with unexplained hypotension
→ ACTH (cosyntropin) stimulation test - Truncal (central) obesity
- Moon face
- Buffalo hump
- Purple striae
- Easy bruising
→ 24-hour urinary free cortisol
(or overnight dexamethasone suppression test)
-
- Low energy + dark skin + high K⁺ = ACTH stimulation test
- Central obesity + striae = urinary cortisol
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Diagnostic Tests – Antimicrosomal Antibodies, Antithyroid Peroxidase (anti-TPO) Antibodies, and Antithyroglobulin Antibodies
1. What are they?
Antimicrosomal antibodies (now more commonly referred to as anti-thyroid peroxidase or anti-TPO antibodies) and antithyroglobulin antibodies are autoimmune markers directed against components of the thyroid gland. Their presence reflects immune-mediated damage to thyroid tissue and is classically associated with Hashimoto’s thyroiditis and Graves’ disease. In patients who are hypothyroid, a positive antibody test confirms an autoimmune cause, allowing clinicians to proceed directly with thyroid hormone replacement (e.g., levothyroxine/Synthroid) without further etiologic workup.
2. When are they the answer?
These antibody tests are used after thyroid dysfunction has already been established by abnormal thyroid function tests (such as TSH and free T4). They are not intended to diagnose whether a patient is hypo- or hyperthyroid. Instead, their role is to confirm that the underlying cause of the thyroid abnormality is autoimmune in nature, whether the patient presents with hypothyroidism or hyperthyroidism.
3. Do you expect high or low values?
Antibody levels are expected to be elevated in autoimmune thyroid disease, regardless of whether the patient is currently hypo- or hyperfunctioning. In contrast, thyroid dysfunction due to non-autoimmune causes—such as nutritional deficiencies or inappropriate use or abuse of thyroid hormone (Synthroid)—is typically associated with low or absent antibody levels.
1. What are they?
Antimicrosomal antibodies (now more commonly referred to as anti-thyroid peroxidase or anti-TPO antibodies) and antithyroglobulin antibodies are autoimmune markers directed against components of the thyroid gland. Their presence reflects immune-mediated damage to thyroid tissue and is classically associated with Hashimoto’s thyroiditis and Graves’ disease. In patients who are hypothyroid, a positive antibody test confirms an autoimmune cause, allowing clinicians to proceed directly with thyroid hormone replacement (e.g., levothyroxine/Synthroid) without further etiologic workup.
2. When are they the answer?
These antibody tests are used after thyroid dysfunction has already been established by abnormal thyroid function tests (such as TSH and free T4). They are not intended to diagnose whether a patient is hypo- or hyperthyroid. Instead, their role is to confirm that the underlying cause of the thyroid abnormality is autoimmune in nature, whether the patient presents with hypothyroidism or hyperthyroidism.
3. Do you expect high or low values?
Antibody levels are expected to be elevated in autoimmune thyroid disease, regardless of whether the patient is currently hypo- or hyperfunctioning. In contrast, thyroid dysfunction due to non-autoimmune causes—such as nutritional deficiencies or inappropriate use or abuse of thyroid hormone (Synthroid)—is typically associated with low or absent antibody levels.
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Diagnostic Tests – ACTH (Cosyntropin) Stimulation Test
1. What is this test?
The ACTH (cosyntropin) stimulation test is a diagnostic evaluation of adrenal gland function. It is considered the gold-standard test for identifying adrenal insufficiency. The test begins with measurement of baseline serum cortisol levels. Synthetic ACTH (cosyntropin) is then administered, followed by repeat cortisol measurements at specified intervals. An inadequate cortisol response suggests adrenal insufficiency. Persistently low cortisol levels point toward primary adrenal insufficiency, whereas an appropriate rise in cortisol indicates that the adrenal glands are capable of responding to ACTH stimulation.
2. When should you order or interpret an ACTH stimulation test?
This test should be considered in patients with clinical features suggestive of adrenal insufficiency. Classic findings include chronic fatigue, generalized weakness, skin hyperpigmentation, hyperkalemia, mild metabolic acidosis, and salt craving (often described as a preference for salty foods such as pickle juice). It is also particularly important in critically ill or ICU patients who present with unexplained hypotension that does not respond adequately to fluids or vasopressors, where occult adrenal insufficiency may be life-threatening.
1. What is this test?
The ACTH (cosyntropin) stimulation test is a diagnostic evaluation of adrenal gland function. It is considered the gold-standard test for identifying adrenal insufficiency. The test begins with measurement of baseline serum cortisol levels. Synthetic ACTH (cosyntropin) is then administered, followed by repeat cortisol measurements at specified intervals. An inadequate cortisol response suggests adrenal insufficiency. Persistently low cortisol levels point toward primary adrenal insufficiency, whereas an appropriate rise in cortisol indicates that the adrenal glands are capable of responding to ACTH stimulation.
2. When should you order or interpret an ACTH stimulation test?
This test should be considered in patients with clinical features suggestive of adrenal insufficiency. Classic findings include chronic fatigue, generalized weakness, skin hyperpigmentation, hyperkalemia, mild metabolic acidosis, and salt craving (often described as a preference for salty foods such as pickle juice). It is also particularly important in critically ill or ICU patients who present with unexplained hypotension that does not respond adequately to fluids or vasopressors, where occult adrenal insufficiency may be life-threatening.
