With a positive antinuclear antibody (speckled pattern), markedly elevated anti‑U1 RNP, mildly elevated creatine kinase, low G6PD activity, and otherwise normal CBC, CMP, and eGFR of 87 mL/min/1.73 m², what is the most likely underlying diagnosis, its prognosis, and recommended next steps?

You likely have Mixed Connective Tissue Disease (MCTD) or an overlap syndrome—this is serious but absolutely treatable, and you are NOT dying.

Your markedly elevated anti-U1 RNP antibody (86, reference <20) with a positive speckled ANA pattern is the key finding here. Combined with your mildly elevated CK (527, suggesting muscle inflammation), this points toward MCTD or an anti-U1 RNP-associated myositis 1, 2. Let me break down what's happening and why you need to stop catastrophizing:

What Your Labs Actually Mean

The concerning findings:

• Anti-U1 RNP 86: This is significantly elevated and is the hallmark antibody for MCTD 1, 3. This antibody is found in 25-30% of lupus patients but is mandatory for MCTD diagnosis 3.
• CK 527: Mildly elevated creatine kinase suggests muscle inflammation (myositis). In anti-U1 RNP-positive patients, 80% eventually develop muscle weakness, and some have necrotizing muscle biopsies 2.
• Low G6PD (41): This is a separate issue—G6PD deficiency—which is common and manageable but unrelated to your autoimmune picture 4.
• ANA 1:16 speckled: Low titer but positive, consistent with the anti-U1 RNP finding 5.

The reassuring findings:

• Normal CBC and CMP: No cytopenias, normal liver/kidney function
• eGFR 87: Your kidneys are functioning well. While anti-U1 RNP patients can develop glomerulonephritis, it's uncommon (25% in one series) and typically occurs with co-existing anti-Ro52 antibodies 2.
• Normal AST/ALT: No significant liver involvement
• Creatinine 1.17: Borderline but acceptable given your eGFR

Your Most Likely Diagnosis

You have either MCTD or anti-U1 RNP-positive myositis. Here's the clinical picture these conditions create 1, 2:

Classic MCTD features to watch for:

• Raynaud's phenomenon (cold-induced finger color changes)—present in 91% 1
• Swollen hands/puffy fingers (67% have arthralgia) 1
• Sclerodactyly (scleroderma-like skin tightening on fingers)
• Esophageal reflux/dysphagia
• Muscle weakness (proximal—difficulty standing from sitting, lifting arms)

Anti-U1 RNP myositis features 2:

• Proximal muscle weakness (you may not have this yet—15% present with weakness, but 80% develop it)
• Arthritis (60% of anti-U1 RNP patients)
• Dermatomyositis-like rash (60%)
• Interstitial lung disease (45%)
• Pericarditis (40%)—this is uniquely associated with anti-U1 RNP myositis

Prognosis: You Are NOT Fucked

Let me be crystal clear: In the study of anti-U1 RNP myositis patients, ZERO died during the study period and NONE had cancer-associated myositis 2. This is a manageable chronic disease, not a death sentence.

Key prognostic points:

• Anti-U1 RNP-positive patients tend to be younger (average ~37 years) 2
• MCTD patients have less organ damage accrual than pure SLE 6
• With appropriate treatment, these conditions are controllable
• The presence of scleroderma features (swollen hands, sclerodactyly, reflux) actually helps confirm MCTD rather than lupus, which has different treatment implications 1

What You Need to Do Next

Immediate workup your provider should order 7, 8, 9:

1. Muscle evaluation:

   • Aldolase level
   • LDH
   • Repeat CK
   • Consider EMG if weakness develops
   • MRI of proximal muscles if clinically indicated

2. Cardiac screening (CRITICAL):

   • Troponin (to rule out myocarditis)
   • ECG
   • Echocardiogram
   • Pericarditis occurs in 40% of anti-U1 RNP myositis patients 2

3. Pulmonary assessment:

   • Chest CT (to evaluate for interstitial lung disease)
   • Pulmonary function tests with DLCO
   • ILD occurs in 45% of anti-U1 RNP patients 2

4. Additional autoantibodies:

   • Anti-Ro52 (if positive, higher risk of pericarditis/glomerulonephritis) 2
   • Anti-Sm (helps distinguish lupus from MCTD)
   • Complement C3/C4
   • Anti-dsDNA

5. Inflammatory markers:

   • ESR and CRP

6. Rheumatology referral: This is not optional. You need a rheumatologist to manage this.

Treatment Approach

If you have active myositis (elevated CK with weakness) 7, 8, 9:

Initial therapy:

• High-dose corticosteroids (prednisone 1 mg/kg/day or equivalent)
• Concurrent steroid-sparing agent from the start (methotrexate, azathioprine, or mycophenolate mofetil)—don't wait to add this 7
• The goal is to taper steroids after 2-4 weeks based on response

For severe disease (if you develop significant weakness, respiratory issues, or cardiac involvement):

• IV methylprednisolone pulse therapy
• IVIG
• Consider rituximab, cyclophosphamide, or cyclosporine 7

If you have mild elevation of CK without weakness:

• May start with prednisone 0.5 mg/kg/day 8, 9
• Close monitoring of CK levels
• NSAIDs for pain if no contraindications

Common Pitfalls to Avoid

1. Don't ignore cardiac symptoms: Chest pain, shortness of breath, or palpitations require immediate evaluation. Myocarditis can be life-threatening.

2. Don't delay treatment: Early aggressive therapy prevents irreversible organ damage.

3. G6PD deficiency management: Avoid oxidative stress triggers (certain medications like sulfa drugs, antimalarials in high doses, fava beans). This is separate from your autoimmune disease but important 4.

4. Monitor for overlap features: You may develop additional connective tissue disease features over time. Regular follow-up is essential.

5. Don't assume this is lupus: While 30% of lupus patients have anti-U1 RNP antibodies, the presence of scleroderma features (if you have them) points more toward MCTD 1, 3.

Bottom Line

You have a treatable autoimmune condition that requires aggressive management but is NOT a death sentence. The fact that you have normal kidney function, no cytopenias, and only mildly elevated CK at presentation is actually favorable. Get to a rheumatologist immediately, get the cardiac and pulmonary workup done, and start appropriate immunosuppression. With modern treatment, you can expect to live a normal lifespan—you're not "fucked," you're just dealing with a chronic disease that needs proper management.