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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 – 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 – 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 — 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.
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Diagnostic Tests – 24-Hour Urine for Metanephrines, Vanillylmandelic Acid (VMA), and Catecholamines
1. For which condition is this the best initial test?
A 24-hour urine collection measuring metanephrines, vanillylmandelic acid (VMA), and catecholamines is the preferred initial screening test for pheochromocytoma. This test detects excess catecholamine production and metabolism, which is characteristic of this catecholamine-secreting tumor.
2. If the test is positive, what is the next best imaging study?
If urinary VMA, metanephrines, or catecholamines are elevated, the next step is anatomical imaging of the adrenal glands. A CT scan or MRI of the adrenal glands is performed to localize the pheochromocytoma.
3. If all standard imaging studies are negative, what should be done next?
If CT and MRI fail to identify a tumor, the next best diagnostic step is metaiodobenzylguanidine (MIBG) scintigraphy. MIBG is a norepinephrine analog that selectively accumulates in adrenal tissue and catecholamine-secreting tumors, making it useful for detecting occult or extra-adrenal pheochromocytomas.
4. This disease is associated with which syndrome?
Pheochromocytoma is classically associated with multiple endocrine neoplasia (MEN) syndromes, particularly MEN type II and MEN type III, as well as other hereditary endocrine tumor syndromes.
5. When do you answer VMA, metanephrines, or catecholamines?
These tests should be considered in patients with episodic or paroxysmal hypertension, especially when accompanied by symptoms such as headaches, palpitations, diaphoresis, flushing, or diarrhea. The episodic nature of symptoms is a key clue pointing toward a catecholamine-secreting tumor.
1. For which condition is this the best initial test?
A 24-hour urine collection measuring metanephrines, vanillylmandelic acid (VMA), and catecholamines is the preferred initial screening test for pheochromocytoma. This test detects excess catecholamine production and metabolism, which is characteristic of this catecholamine-secreting tumor.
2. If the test is positive, what is the next best imaging study?
If urinary VMA, metanephrines, or catecholamines are elevated, the next step is anatomical imaging of the adrenal glands. A CT scan or MRI of the adrenal glands is performed to localize the pheochromocytoma.
3. If all standard imaging studies are negative, what should be done next?
If CT and MRI fail to identify a tumor, the next best diagnostic step is metaiodobenzylguanidine (MIBG) scintigraphy. MIBG is a norepinephrine analog that selectively accumulates in adrenal tissue and catecholamine-secreting tumors, making it useful for detecting occult or extra-adrenal pheochromocytomas.
4. This disease is associated with which syndrome?
Pheochromocytoma is classically associated with multiple endocrine neoplasia (MEN) syndromes, particularly MEN type II and MEN type III, as well as other hereditary endocrine tumor syndromes.
5. When do you answer VMA, metanephrines, or catecholamines?
These tests should be considered in patients with episodic or paroxysmal hypertension, especially when accompanied by symptoms such as headaches, palpitations, diaphoresis, flushing, or diarrhea. The episodic nature of symptoms is a key clue pointing toward a catecholamine-secreting tumor.
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Diagnostic Tests – 24-Hour Urine for Metanephrines, Vanillylmandelic Acid (VMA), and Catecholamines
1. For which condition is this the best initial test?
A 24-hour urine collection for metanephrines, VMA, and catecholamines is the best initial screening test for pheochromocytoma. This tumor secretes excessive catecholamines, and their metabolites are more reliably detected over a full 24-hour period than with spot measurements.
2. If the test is positive, what is the next best imaging study?
If urinary metanephrines, VMA, or catecholamines are elevated, the next step is localization of the tumor. The best initial imaging studies are a CT scan or MRI of the adrenal glands, which can identify most adrenal pheochromocytomas.
3. If all imaging studies are negative, what should be done next?
If CT and MRI do not reveal a tumor, the next diagnostic step is metaiodobenzylguanidine (MIBG) scintigraphy. MIBG is a norepinephrine analog that is selectively taken up by catecholamine-secreting cells, making it useful for detecting occult, extra-adrenal, or metastatic pheochromocytomas.
4. This disease is associated with which syndrome?
Pheochromocytoma is classically associated with multiple endocrine neoplasia (MEN) syndromes, particularly MEN type II and MEN type III, and should raise suspicion for an underlying genetic endocrine disorder.
5. When do you answer VMA, metanephrines, or catecholamines?
These tests are indicated when a patient presents with episodic or paroxysmal hypertension, especially when accompanied by symptoms such as flushing, diarrhea, headaches, palpitations, or diaphoresis. The intermittent nature of hypertension and autonomic symptoms is a key clinical clue pointing toward a catecholamine-secreting tumor.
1. For which condition is this the best initial test?
A 24-hour urine collection for metanephrines, VMA, and catecholamines is the best initial screening test for pheochromocytoma. This tumor secretes excessive catecholamines, and their metabolites are more reliably detected over a full 24-hour period than with spot measurements.
2. If the test is positive, what is the next best imaging study?
If urinary metanephrines, VMA, or catecholamines are elevated, the next step is localization of the tumor. The best initial imaging studies are a CT scan or MRI of the adrenal glands, which can identify most adrenal pheochromocytomas.
3. If all imaging studies are negative, what should be done next?
If CT and MRI do not reveal a tumor, the next diagnostic step is metaiodobenzylguanidine (MIBG) scintigraphy. MIBG is a norepinephrine analog that is selectively taken up by catecholamine-secreting cells, making it useful for detecting occult, extra-adrenal, or metastatic pheochromocytomas.
4. This disease is associated with which syndrome?
Pheochromocytoma is classically associated with multiple endocrine neoplasia (MEN) syndromes, particularly MEN type II and MEN type III, and should raise suspicion for an underlying genetic endocrine disorder.
5. When do you answer VMA, metanephrines, or catecholamines?
These tests are indicated when a patient presents with episodic or paroxysmal hypertension, especially when accompanied by symptoms such as flushing, diarrhea, headaches, palpitations, or diaphoresis. The intermittent nature of hypertension and autonomic symptoms is a key clinical clue pointing toward a catecholamine-secreting tumor.
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Diagnostic Tests – Pro-Insulin
1. What is this test for?
The pro-insulin assay is used to help determine the cause of hypoglycemia, specifically to differentiate endogenous insulin overproduction from other etiologies. It is particularly useful for distinguishing insulinoma from autoimmune or exogenous causes of hypoglycemia.
2. What do high levels of pro-insulin represent?
Elevated pro-insulin levels strongly suggest the presence of an insulinoma. Insulinomas secrete excessive amounts of insulin and incompletely processed insulin precursors, leading to disproportionately high pro-insulin concentrations compared with normal physiology.
3. When do you answer pro-insulin levels?
You should choose a pro-insulin level in cases of recurrent fasting hypoglycemia, especially when the patient presents with symptoms such as weakness, confusion, diaphoresis, or tremors and laboratory findings show inappropriately elevated C-peptide levels. This combination indicates endogenous insulin secretion and points toward insulinoma as the underlying diagnosis rather than surreptitious insulin use or autoimmune hypoglycemia.
1. What is this test for?
The pro-insulin assay is used to help determine the cause of hypoglycemia, specifically to differentiate endogenous insulin overproduction from other etiologies. It is particularly useful for distinguishing insulinoma from autoimmune or exogenous causes of hypoglycemia.
2. What do high levels of pro-insulin represent?
Elevated pro-insulin levels strongly suggest the presence of an insulinoma. Insulinomas secrete excessive amounts of insulin and incompletely processed insulin precursors, leading to disproportionately high pro-insulin concentrations compared with normal physiology.
3. When do you answer pro-insulin levels?
You should choose a pro-insulin level in cases of recurrent fasting hypoglycemia, especially when the patient presents with symptoms such as weakness, confusion, diaphoresis, or tremors and laboratory findings show inappropriately elevated C-peptide levels. This combination indicates endogenous insulin secretion and points toward insulinoma as the underlying diagnosis rather than surreptitious insulin use or autoimmune hypoglycemia.
