Which laboratory studies are recommended for an adult presenting with fixed non‑bizarre delusions to diagnose delusional disorder?

Laboratory Testing for Delusional Disorder

No routine laboratory tests are mandatory for diagnosing delusional disorder in alert, cooperative adults with normal vital signs and a noncontributory history and physical examination—diagnostic evaluation should be directed by clinical findings rather than reflexive testing. 1

Core Principle: History and Physical Examination Drive Testing

The diagnosis of delusional disorder is fundamentally clinical, based on DSM-5 or ICD-10 criteria requiring at least 1 month of non-bizarre delusions without significant functional impairment or other psychopathology. 2 Laboratory testing serves only to exclude medical mimics, not to confirm the psychiatric diagnosis itself. 1

When Laboratory Testing Is NOT Indicated

• Alert, cooperative patients with normal vital signs, noncontributory history, and normal physical examination do not require routine laboratory screening. 1
• Studies demonstrate that routine laboratory panels have extremely low yield (0.8-1.8%) when history and physical examination are unremarkable, with false-positive results occurring 8 times more frequently than true positives. 1
• History and physical examination identify 80-94% of clinically significant medical causes when present. 1

Risk Stratification: Who Requires Laboratory Evaluation

Obtain selective laboratory testing only in these high-risk groups: 1

• Elderly patients (age >65 years) 1
• First psychiatric presentation (no prior psychiatric history) 1
• Substance abuse history 1
• New or preexisting medical complaints 1
• Lower socioeconomic status 1
• Abnormal vital signs (fever, tachycardia, hypertension, hypotension) 1
• Abnormal physical examination findings (focal neurological deficits, cognitive impairment) 1

Selective Laboratory Panel (When Clinically Indicated)

If risk factors are present, obtain these tests based on specific clinical suspicion: 1

Metabolic and Endocrine Screening

• Thyroid function tests (TSH, free T4) if affective symptoms present, as thyroid disease can cause delusions in affective disorders 1
• Serum glucose if diabetes suspected or symptoms of hypo/hyperglycemia 3
• Complete metabolic panel (electrolytes, BUN, creatinine) if renal disease, medication effects, or metabolic derangement suspected 3
• Serum calcium and magnesium if hypocalcemia/hypomagnesemia risk factors present (alcoholism, renal disease, certain medications) 4

Infection and Inflammatory Markers

• Complete blood count (CBC) if infection suspected based on fever or immunocompromised status 4
• Urinalysis particularly in elderly women or if urinary symptoms present 1
• HIV testing and syphilis serology (RPR/VDRL) based on risk factors 4

Toxicology

• Urine drug screen if substance use suspected clinically, though routine screening in the absence of clinical suspicion is controversial 1

Geriatric-Specific Testing

• Vitamin B12 level in elderly patients, despite low yield 1
• ECG, chest radiograph in psychogeriatric population 1

Neuroimaging: When to Order

Brain imaging (CT or MRI) is NOT routinely indicated for delusional disorder diagnosis. 1

Obtain neuroimaging only if: 1, 4

• History of head trauma
• New or worsening headaches
• Focal neurological signs or deficits
• First-episode psychosis with atypical features
• Altered level of consciousness
• Abnormal cognitive examination

Advanced Testing (Rarely Needed)

These tests are NOT part of routine evaluation: 1

• Lumbar puncture with CSF analysis: Only if CNS infection suspected (fever, immunocompromised, meningeal signs) 1, 4
• EEG: Only if seizure disorder suspected 4
• Sleep-deprived EEG: Not indicated 1

Critical Pitfalls to Avoid

• Do not order extensive routine laboratory batteries reflexively—this leads to false positives and unnecessary follow-up testing 1
• Do not assume psychiatric etiology without proper medical screening in high-risk groups (elderly, first presentation, substance abuse, abnormal vitals) 1, 3
• Do not confuse delusional disorder with delirium—delirium requires acute onset, fluctuating course, inattention, and altered consciousness, necessitating comprehensive medical workup 1
• Do not order tests that were not predicted by history and physical examination—only 2 of 463 abnormal results in one study were unpredicted, and neither required treatment 1

Algorithmic Approach

1. Complete thorough history and physical examination first 1
2. Assess for high-risk features (elderly, first episode, substance use, medical complaints, abnormal vitals/exam) 1
3. If NO risk factors present: No laboratory testing required 1
4. If risk factors present: Order selective tests based on specific clinical suspicion 1
5. Reserve neuroimaging and advanced testing for focal deficits, trauma, or diagnostic uncertainty 1, 4