What follow-up is needed for a 58-year-old male with hypertension (HTN), hyperlipidemia, erythrocytosis (elevated hemoglobin 18.5, hematocrit 58.1, red blood cell count 7.01), and impaired renal function (creatinine 1.48, estimated glomerular filtration rate 55)?

Immediate Workup for Erythrocytosis and Renal Impairment

This patient requires urgent evaluation for secondary erythrocytosis, likely related to his renal disease, with immediate JAK2 mutation testing, serum erythropoietin level, and renal imaging to exclude polycythemia vera, renal cysts, or renal cell carcinoma. 1, 2, 3

Priority 1: Distinguish Primary from Secondary Erythrocytosis

The hemoglobin of 18.5 g/dL and hematocrit of 58.1% represent significant erythrocytosis requiring immediate investigation. 2, 3

Order these tests immediately:

• JAK2 V617F mutation testing - This is the first-line test to exclude polycythemia vera, as JAK2 mutations are present in nearly all cases of PV. 2, 3, 4
• Serum erythropoietin (EPO) level - A low EPO suggests primary polycythemia vera, while normal or elevated EPO indicates secondary erythrocytosis. 2, 3, 4
• Complete blood count with differential - Evaluate white blood cell and platelet counts, as elevations suggest PV rather than secondary causes. 5, 2

Priority 2: Evaluate for Renal Causes of Erythrocytosis

Given the eGFR of 55 mL/min/1.73 m² (CKD stage 3a), renal pathology is the most likely driver of secondary erythrocytosis. 6, 7, 4

Obtain renal imaging immediately:

• Renal ultrasound with Doppler - Screen for renal cysts, acquired cystic kidney disease, or renal masses that can produce EPO. 1, 6, 4
• CT abdomen/pelvis with contrast (if renal function permits) or MRI - If ultrasound shows cysts or masses, further characterization is essential as renal cell carcinoma and complex cysts are EPO-secreting tumors. 1, 2, 4

Acquired cystic kidney disease in CKD patients can cause spontaneous erythrocytosis through EPO production, even with normal serum EPO levels initially. 6, 4 This condition carries risks of malignancy and vascular events from elevated hemoglobin. 6

Priority 3: Comprehensive Renal Function Assessment

The creatinine of 1.48 mg/dL and eGFR of 55 mL/min/1.73 m² require full CKD evaluation. 5, 1

Complete these tests within 1 week:

• Urine albumin-to-creatinine ratio (UACR) on spot morning urine - Essential for CKD staging and cardiovascular risk stratification. 5, 1
• Complete urinalysis with microscopy - Evaluate for active sediment (RBC casts, dysmorphic RBCs suggesting glomerulonephritis) versus bland sediment typical of hypertensive nephrosclerosis. 1
• Serum electrolytes including potassium - Critical before initiating or adjusting ACE inhibitors/ARBs. 1
• Fasting lipid panel and hemoglobin A1C - Assess cardiovascular risk and screen for diabetes, which is common in CKD patients. 5

Priority 4: Screen for Secondary Hypertension

The combination of erythrocytosis, hypertension, and renal impairment raises concern for renovascular disease or other secondary causes. 5, 1, 4

Consider these evaluations:

• Renovascular disease screening - Renal Doppler ultrasound, CT angiography, or MR angiography if patient has risk factors (age >50, smoking, peripheral arterial disease, unexplained renal insufficiency). 1
• Primary aldosteronism screening - Plasma aldosterone concentration and plasma renin activity if hypertension is difficult to control. 5

Priority 5: Monitoring Schedule

Immediate follow-up (within 2 weeks):

• Review JAK2 mutation, EPO level, and imaging results. 2, 3
• Repeat CBC to confirm erythrocytosis is persistent. 5, 2

Short-term monitoring (monthly for 3 months):

• Serum creatinine, eGFR, and UACR - More frequent testing is recommended for eGFR 30-60 mL/min/1.73 m². 5, 1
• Blood pressure monitoring - Home BP measurements using validated devices with proper technique. 5

Long-term monitoring (every 6-12 months):

• Annual monitoring of creatinine, eGFR, and UACR at minimum for CKD stage 3a. 5, 1
• CBC to monitor hemoglobin trends. 5
• Fasting glucose or A1C, lipid panel, TSH. 5

Priority 6: Nephrology Referral

Refer to nephrology now given the combination of unexplained erythrocytosis with CKD stage 3a, as this represents uncertainty about the etiology of kidney disease and potential for rapidly progressive disease. 1

Critical Pitfalls to Avoid

• Do not assume this is simple dehydration - The RBC count of 7.01 million/μL indicates true erythrocytosis, not hemoconcentration. 2, 3
• Do not delay imaging - EPO-secreting renal tumors can present with normal EPO levels initially, and acquired cystic disease increases malignancy risk. 6, 4
• Do not overlook cardiovascular risk - Erythrocytosis with hematocrit >58% significantly increases thrombotic risk and requires urgent management regardless of etiology. 6, 7
• Do not start phlebotomy before diagnosis - Wait for JAK2 and EPO results to guide appropriate therapy, as management differs between PV and secondary causes. 2, 3