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Diagnostic Tests – DEXA Scanning (Dual-Energy X-ray Absorptiometry)
1. What does it show?
A DEXA scan measures bone mineral density (BMD) and is the standard test used to diagnose osteoporosis. It compares a patient’s bone density to that of a healthy young adult population and reports results as T-scores, allowing assessment of fracture risk and bone loss severity.
2. When do you answer a DEXA scan?
You should select DEXA scanning when evaluating patients at high risk for osteoporosis, including:
Exam pearl:
Any condition causing increased bone resorption or decreased bone formation → think DEXA scan.
3. What is the treatment?
Management of osteoporosis includes:
1. What does it show?
A DEXA scan measures bone mineral density (BMD) and is the standard test used to diagnose osteoporosis. It compares a patient’s bone density to that of a healthy young adult population and reports results as T-scores, allowing assessment of fracture risk and bone loss severity.
2. When do you answer a DEXA scan?
You should select DEXA scanning when evaluating patients at high risk for osteoporosis, including:
- Postmenopausal women
- Patients with high cortisol states, such as:
- Cushing’s syndrome
- Chronic exogenous steroid use
- Patients with hyperthyroidism
- Patients with hyperparathyroidism
Exam pearl:
Any condition causing increased bone resorption or decreased bone formation → think DEXA scan.
3. What is the treatment?
Management of osteoporosis includes:
- Estrogen therapy (selected postmenopausal patients)
- Bisphosphonates (first-line therapy for most patients)
- Selective estrogen receptor modulators (SERMs) such as:
- Raloxifene
- TamoxifeN
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Diagnostic Tests – Rheumatoid Factor (RF)
1. What disease is it associated with?
Rheumatoid factor (RF) is most commonly associated with rheumatoid arthritis (RA). It is present in over 70% of patients with RA, making it a useful supportive test. However, RF is not specific for RA and may also be positive in other autoimmune diseases, chronic infections, and even in some healthy older adults.
2. When do you answer rheumatoid factor?
You should select RF testing when the clinical scenario describes:
Symmetric small-joint inflammatory arthritis with morning stiffness → think rheumatoid arthritis → order rheumatoid factor (often along with anti-CCP antibodies).
1. What disease is it associated with?
Rheumatoid factor (RF) is most commonly associated with rheumatoid arthritis (RA). It is present in over 70% of patients with RA, making it a useful supportive test. However, RF is not specific for RA and may also be positive in other autoimmune diseases, chronic infections, and even in some healthy older adults.
2. When do you answer rheumatoid factor?
You should select RF testing when the clinical scenario describes:
- A woman with symmetric inflammatory arthritis
- Morning stiffness that improves with activity
- Involvement of the wrists, metacarpophalangeal (MCP) joints, and proximal interphalangeal (PIP) joints
- Laboratory evidence of inflammation, such as:
- Elevated ESR
- Synovial fluid WBC count between 2,000 and 20,000 cells/µL (inflammatory range)
Symmetric small-joint inflammatory arthritis with morning stiffness → think rheumatoid arthritis → order rheumatoid factor (often along with anti-CCP antibodies).
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Diagnostic Tests – Allergy Testing (Skin Prick / Intradermal Testing)
1. For which clinical scenario is this test used?
Allergy testing involves subdermal or skin exposure to small amounts of suspected allergens, followed by observation for a localized hypersensitivity reaction. It is used when a patient has allergic symptoms and the specific trigger is unclear, such as:
2. When is allergy testing contraindicated?
Allergy testing is contraindicated in patients with:
High anaphylaxis risk or unstable asthma → do NOT perform skin allergy testing
3. What medication must be stopped before allergy testing?
Beta blockers (e.g., propranolol) must be stopped prior to allergy testing or desensitization when possible.
Why?
Allergy testing + beta blocker = dangerous combination
1. For which clinical scenario is this test used?
Allergy testing involves subdermal or skin exposure to small amounts of suspected allergens, followed by observation for a localized hypersensitivity reaction. It is used when a patient has allergic symptoms and the specific trigger is unclear, such as:
- Allergic rhinitis (runny nose, sneezing, watery eyes)
- Suspected allergies to:
- Foods
- Pets
- Aeroallergens (pollens, dust mites, molds)
- Venoms (bees, wasps)
2. When is allergy testing contraindicated?
Allergy testing is contraindicated in patients with:
- A high risk of anaphylaxis based on prior severe reactions
- Uncontrolled or severe asthma, due to increased risk of life-threatening reactions during testing
High anaphylaxis risk or unstable asthma → do NOT perform skin allergy testing
3. What medication must be stopped before allergy testing?
Beta blockers (e.g., propranolol) must be stopped prior to allergy testing or desensitization when possible.
Why?
- If anaphylaxis occurs, epinephrine is the life-saving treatment
- Beta blockers blunt the response to epinephrine, making treatment of anaphylaxis less effective
Allergy testing + beta blocker = dangerous combination
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Diagnostic Tests – C-ANCA (Antineutrophil Cytoplasmic Antibody)
1. What is this?
C-ANCA is a serologic test used to diagnose granulomatosis with polyangiitis (GPA), formerly known as Wegener’s granulomatosis. It is the best initial test to make a specific diagnosis of this vasculitic disorder. The antibodies are directed against proteinase-3 (PR3) in neutrophils.
2. How is it done?
C-ANCA is a blood test analyzed using indirect immunofluorescence microscopy. The characteristic cytoplasmic (C-ANCA) staining pattern reflects antibodies targeting PR3, which is highly associated with granulomatosis with polyangiitis.
3. When do you answer C-ANCA?
You should select C-ANCA when a patient presents with a classic triad of involvement:
ENT symptoms + lung disease + renal findings → C-ANCA
4. What is the most accurate diagnostic test?
The most accurate (definitive) diagnostic test for granulomatosis with polyangiitis is a tissue biopsy, typically from:
1. What is this?
C-ANCA is a serologic test used to diagnose granulomatosis with polyangiitis (GPA), formerly known as Wegener’s granulomatosis. It is the best initial test to make a specific diagnosis of this vasculitic disorder. The antibodies are directed against proteinase-3 (PR3) in neutrophils.
2. How is it done?
C-ANCA is a blood test analyzed using indirect immunofluorescence microscopy. The characteristic cytoplasmic (C-ANCA) staining pattern reflects antibodies targeting PR3, which is highly associated with granulomatosis with polyangiitis.
3. When do you answer C-ANCA?
You should select C-ANCA when a patient presents with a classic triad of involvement:
- Upper respiratory tract disease: chronic sinusitis, otitis media
- Lower respiratory tract disease: cough, hemoptysis, sputum production, abnormal chest X-ray not responsive to antibiotics
- Renal disease: hematuria, red blood cell casts
ENT symptoms + lung disease + renal findings → C-ANCA
4. What is the most accurate diagnostic test?
The most accurate (definitive) diagnostic test for granulomatosis with polyangiitis is a tissue biopsy, typically from:
- Lung, or
- Kidney
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Diagnostic Tests – Anti–Scl-70 (Anti–Topoisomerase I) Antibodies
1. What is this?
Anti–Scl-70 antibodies are autoantibodies directed against topoisomerase I, a nuclear enzyme. They are associated with systemic sclerosis, particularly the diffuse cutaneous form. Their presence supports the diagnosis and helps with risk stratification.
2. When do you answer anti–Scl-70 antibodies?
Order anti–Scl-70 when the clinical picture suggests diffuse systemic sclerosis, such as:
Exam pearl: Diffuse scleroderma features → think anti–Scl-70.
3. How would a negative test affect management?
A negative anti–Scl-70 does not rule out systemic sclerosis because the test has low sensitivity (~30%).
If clinical suspicion remains high, management and further evaluation should continue despite a negative result.
4. What is the patient at risk for if anti–Scl-70 is positive?
A positive anti–Scl-70 is most strongly associated with severe pulmonary involvement, especially:
This finding signals a higher-risk disease course and warrants close pulmonary monitoring.
High-yield contrast
1. What is this?
Anti–Scl-70 antibodies are autoantibodies directed against topoisomerase I, a nuclear enzyme. They are associated with systemic sclerosis, particularly the diffuse cutaneous form. Their presence supports the diagnosis and helps with risk stratification.
2. When do you answer anti–Scl-70 antibodies?
Order anti–Scl-70 when the clinical picture suggests diffuse systemic sclerosis, such as:
- Polyarthralgia
- Raynaud phenomenon
- Thick, tight (“hide-bound”) skin
- Dysphagia and GI hypomotility
- Evidence of pulmonary and/or renal involvement
Exam pearl: Diffuse scleroderma features → think anti–Scl-70.
3. How would a negative test affect management?
A negative anti–Scl-70 does not rule out systemic sclerosis because the test has low sensitivity (~30%).
If clinical suspicion remains high, management and further evaluation should continue despite a negative result.
4. What is the patient at risk for if anti–Scl-70 is positive?
A positive anti–Scl-70 is most strongly associated with severe pulmonary involvement, especially:
- Interstitial lung disease
- Pulmonary fibrosis
This finding signals a higher-risk disease course and warrants close pulmonary monitoring.
High-yield contrast
- Limited systemic sclerosis (CREST) → Anticentromere antibodies
- Diffuse systemic sclerosis → Anti–Scl-70 antibodies
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Diagnostic Tests – Anti-Ro (SSA) and Anti-La (SSB) Antibodies
1. What diseases are they associated with?
Anti-Ro (SSA) and Anti-La (SSB) antibodies are most strongly associated with Sjögren’s syndrome. Patients with Sjögren’s syndrome classically have:
These antibodies may also be seen in other connective tissue diseases, but their strongest association is with Sjögren’s syndrome.
2. When are they the answer?
You should select anti-Ro and anti-La antibody testing when the clinical scenario describes the classic sicca triad, which includes:
Dry eyes + dry mouth + arthritis → think Sjögren’s syndrome → order anti-Ro / anti-La antibodies.
1. What diseases are they associated with?
Anti-Ro (SSA) and Anti-La (SSB) antibodies are most strongly associated with Sjögren’s syndrome. Patients with Sjögren’s syndrome classically have:
- Positive ANA
- Anti-Ro and/or Anti-La antibodies
- Often a positive rheumatoid factor
These antibodies may also be seen in other connective tissue diseases, but their strongest association is with Sjögren’s syndrome.
2. When are they the answer?
You should select anti-Ro and anti-La antibody testing when the clinical scenario describes the classic sicca triad, which includes:
- Dry eyes (keratoconjunctivitis sicca)
- Dry mouth (xerostomia)
- Arthritis or arthralgias
Dry eyes + dry mouth + arthritis → think Sjögren’s syndrome → order anti-Ro / anti-La antibodies.
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Diagnostic Tests – Aldolase
1. What is it associated with?
Aldolase is an enzyme released from damaged skeletal muscle. It is elevated in inflammatory myopathies, particularly:
2. When do you answer aldolase?
You should choose aldolase (or CPK) as the best initial diagnostic test when the clinical vignette suggests an inflammatory myopathy, such as:
Exam pearl:
Proximal muscle weakness + dermatomyositis rash → check aldolase or CPK first
3. What is the most accurate test?
While aldolase is useful for initial screening, the most accurate diagnostic tests for polymyositis and dermatomyositis are:
1. What is it associated with?
Aldolase is an enzyme released from damaged skeletal muscle. It is elevated in inflammatory myopathies, particularly:
- Polymyositis
- Dermatomyositis
2. When do you answer aldolase?
You should choose aldolase (or CPK) as the best initial diagnostic test when the clinical vignette suggests an inflammatory myopathy, such as:
- An older woman with proximal muscle weakness
- Difficulty rising from a chair or lifting arms
- Associated myalgias
- Characteristic skin findings, including:
- Heliotrope rash
- Gottron’s papules
Exam pearl:
Proximal muscle weakness + dermatomyositis rash → check aldolase or CPK first
3. What is the most accurate test?
While aldolase is useful for initial screening, the most accurate diagnostic tests for polymyositis and dermatomyositis are:
- Electromyography (EMG)
- Muscle biopsy
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Diagnostic Tests – 17-Hydroxyprogesterone Level
1. What is this test?
The 17-hydroxyprogesterone (17-OHP) level is a screening blood test used to diagnose congenital adrenal hyperplasia (CAH), most commonly due to 21-hydroxylase deficiency. In this condition, impaired cortisol synthesis leads to accumulation of 17-hydroxyprogesterone, which is shunted into androgen production.
2. When do you answer it?
You should select a 17-hydroxyprogesterone level when the clinical scenario suggests excess androgen production or impaired adrenal steroid synthesis, including:
In exam settings, these presentations strongly point toward 21-hydroxylase deficiency, making 17-hydroxyprogesterone the best initial diagnostic test.
1. What is this test?
The 17-hydroxyprogesterone (17-OHP) level is a screening blood test used to diagnose congenital adrenal hyperplasia (CAH), most commonly due to 21-hydroxylase deficiency. In this condition, impaired cortisol synthesis leads to accumulation of 17-hydroxyprogesterone, which is shunted into androgen production.
2. When do you answer it?
You should select a 17-hydroxyprogesterone level when the clinical scenario suggests excess androgen production or impaired adrenal steroid synthesis, including:
- Female infant with ambiguous genitalia (simple virilizing form)
- Infant with hyponatremia, hyperkalemia, and hypotension (salt-wasting crisis)
- Young woman with hirsutism, acne, and irregular menses
- Child with precocious puberty
In exam settings, these presentations strongly point toward 21-hydroxylase deficiency, making 17-hydroxyprogesterone the best initial diagnostic test.
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Diagnostic Tests – Fungal Culture
1. For which clinical scenario(s) is this the best (most accurate) test?
A fungal culture is the most accurate confirmatory test for superficial fungal infections, including:
It allows definitive identification of the fungal organism and is especially useful when the diagnosis is uncertain or treatment has failed.
2. When is this test the best initial step?
Never.
Although fungal culture is highly accurate, it is not the best initial diagnostic test. Superficial fungal infections are primarily diagnosed by:
3. What is the management?
Treatment depends on the location of the fungal infection:
Suspected fungal infection → KOH prep first
Need definitive identification or refractory case → Fungal culture
1. For which clinical scenario(s) is this the best (most accurate) test?
A fungal culture is the most accurate confirmatory test for superficial fungal infections, including:
- Tinea infections (corporis, cruris, pedis)
- Onychomycosis (nail infections)
- Fungal vaginitis
It allows definitive identification of the fungal organism and is especially useful when the diagnosis is uncertain or treatment has failed.
2. When is this test the best initial step?
Never.
Although fungal culture is highly accurate, it is not the best initial diagnostic test. Superficial fungal infections are primarily diagnosed by:
- Clinical appearance
- KOH preparation (to visualize hyphae or yeast)
3. What is the management?
Treatment depends on the location of the fungal infection:
- Require oral antifungals:
- Onychomycosis
- Tinea capitis
- Medications: terbinafine or itraconazole
- Treated with topical antifungals:
- Most other cutaneous fungal infections
- Medications: ketoconazole, clotrimazole, nystatin, ciclopirox
Suspected fungal infection → KOH prep first
Need definitive identification or refractory case → Fungal culture
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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
