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Diagnostic Tests – Metaiodobenzylguanidine (MIBG) Scan
1. What disease is this test for?
The metaiodobenzylguanidine (MIBG) scan is used to detect pheochromocytoma, particularly when the tumor is occult or not visualized on standard imaging. It is not a screening test; instead, it is used after biochemical confirmation of catecholamine excess (elevated metanephrines, catecholamines, or VMA) when CT or MRI fails to localize the tumor.
2. How is the test done?
MIBG, a compound structurally similar to norepinephrine, is injected intravenously. Because pheochromocytomas arise from adrenergic tissue, they actively take up MIBG. Nuclear imaging is then performed to visualize areas of tracer accumulation, allowing identification of adrenal or extra-adrenal catecholamine-secreting tumors.
3. When do you answer an MIBG scan?
You should select an MIBG scan in a patient who has:
This test is essential because a pheochromocytoma cannot be surgically cured unless it is accurately localized.
1. What disease is this test for?
The metaiodobenzylguanidine (MIBG) scan is used to detect pheochromocytoma, particularly when the tumor is occult or not visualized on standard imaging. It is not a screening test; instead, it is used after biochemical confirmation of catecholamine excess (elevated metanephrines, catecholamines, or VMA) when CT or MRI fails to localize the tumor.
2. How is the test done?
MIBG, a compound structurally similar to norepinephrine, is injected intravenously. Because pheochromocytomas arise from adrenergic tissue, they actively take up MIBG. Nuclear imaging is then performed to visualize areas of tracer accumulation, allowing identification of adrenal or extra-adrenal catecholamine-secreting tumors.
3. When do you answer an MIBG scan?
You should select an MIBG scan in a patient who has:
- Symptoms suggestive of pheochromocytoma (e.g., palpitations, episodic hypertension, intermittent tachycardia, diaphoresis)
- Positive biochemical testing (elevated catecholamines, metanephrines, or VMA)
- Negative or nondiagnostic CT or MRI
This test is essential because a pheochromocytoma cannot be surgically cured unless it is accurately localized.
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Diagnostic Tests – 5-HIAA (5-Hydroxyindoleacetic Acid)
1. What is this test?
A 24-hour urine 5-HIAA level measures 5-hydroxyindoleacetic acid, the primary end-product of serotonin metabolism. It is the best initial diagnostic test for carcinoid syndrome, reflecting excess serotonin production by neuroendocrine tumors.
2. When do you answer a 5-HIAA level?
You should choose a 5-HIAA test when a patient presents with episodic symptoms suggestive of carcinoid syndrome, including:
The combination of flushing plus diarrhea is particularly high-yield for test questions.
3. What is the most accurate diagnostic test?
The most accurate diagnostic test for carcinoid syndrome is a biopsy with histologic confirmation. While CT and MRI scans can help localize tumors or metastases, they are not as definitive as tissue diagnosis obtained through biopsy.
1. What is this test?
A 24-hour urine 5-HIAA level measures 5-hydroxyindoleacetic acid, the primary end-product of serotonin metabolism. It is the best initial diagnostic test for carcinoid syndrome, reflecting excess serotonin production by neuroendocrine tumors.
2. When do you answer a 5-HIAA level?
You should choose a 5-HIAA test when a patient presents with episodic symptoms suggestive of carcinoid syndrome, including:
- Recurrent watery diarrhea
- Unexplained flushing
- Wheezing or bronchospasm
- Episodic hypotension
The combination of flushing plus diarrhea is particularly high-yield for test questions.
3. What is the most accurate diagnostic test?
The most accurate diagnostic test for carcinoid syndrome is a biopsy with histologic confirmation. While CT and MRI scans can help localize tumors or metastases, they are not as definitive as tissue diagnosis obtained through biopsy.
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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 – 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 – Excision Skin Biopsy
1. For which clinical scenario(s) is this diagnostic tool indicated?
An excision skin biopsy is a diagnostic technique in which the entire skin lesion is removed with full thickness, ideally in one intact specimen. It is indicated when there is high clinical suspicion for malignancy, particularly when accurate assessment of lesion depth, margins, and architecture is required for diagnosis and staging.
2. For which condition is this test always the right answer?
An excision biopsy is always the correct diagnostic test for suspected malignant melanoma. Melanoma requires complete removal of the lesion to allow proper evaluation of Breslow depth, which is critical for staging and prognosis. Partial biopsies (such as shave or punch biopsies) are inadequate because they may prevent accurate staging and risk underestimating tumor depth.
Key exam pearl: If melanoma is suspected, excision biopsy is the answer—always.
1. For which clinical scenario(s) is this diagnostic tool indicated?
An excision skin biopsy is a diagnostic technique in which the entire skin lesion is removed with full thickness, ideally in one intact specimen. It is indicated when there is high clinical suspicion for malignancy, particularly when accurate assessment of lesion depth, margins, and architecture is required for diagnosis and staging.
2. For which condition is this test always the right answer?
An excision biopsy is always the correct diagnostic test for suspected malignant melanoma. Melanoma requires complete removal of the lesion to allow proper evaluation of Breslow depth, which is critical for staging and prognosis. Partial biopsies (such as shave or punch biopsies) are inadequate because they may prevent accurate staging and risk underestimating tumor depth.
Key exam pearl: If melanoma is suspected, excision biopsy is the answer—always.
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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 – 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 — 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
