Heart Failure: The presence of pericardial effusion can lead to cardiac tamponade or heart failure, which can cause fluid overload and subsequent urinary frequency due to increased renal perfusion and diuretic effects. The negative urinalysis and urine culture rule out urinary tract infection as a cause of the symptoms.

Diuretic Use: If the patient is on diuretics for heart failure or other conditions, this could exacerbate urinary frequency.
Age-Related Detrusor Overactivity: Overactive bladder is common in the elderly due to decreased bladder capacity, detrusor overactivity, and other age-related changes.
Polypharmacy: Elderly patients often take multiple medications, some of which (e.g., diuretics, sedatives, and certain antidepressants) can affect bladder function and lead to urinary frequency.

Urinary Retention with Overflow Incontinence: Although the urinalysis and urine culture are negative, urinary retention can sometimes present with frequency and must be ruled out, especially in elderly patients with possible neurological or obstructive causes.
Malignancy (e.g., Bladder or Urethral Cancer): Although less likely with negative urinalysis, certain malignancies can cause urinary frequency and must be considered, especially if other symptoms like hematuria or weight loss are present.
Neurological Disorders (e.g., Stroke, Multiple Sclerosis, Spinal Cord Injury): These conditions can affect bladder control and cause urinary frequency.

Autoimmune Disorders (e.g., Sjögren's Syndrome): Can cause interstitial cystitis or other urinary symptoms due to autoimmune inflammation.
Endocrine Disorders (e.g., Diabetes Insipidus, Hypercalcemia): Certain endocrine conditions can lead to polyuria and subsequent urinary frequency.
Genitourinary Fistula: An abnormal connection between the urinary tract and another organ or the skin, which can cause continuous urinary leakage and frequency.