Baseline Laboratory Tests for CKD Stage 3b
For a patient with CKD stage 3b (eGFR 30-44 ml/min/1.73 m²), order the following baseline labs: serum creatinine with eGFR, urine albumin-to-creatinine ratio (ACR), complete blood count (CBC), serum calcium, phosphorus, intact PTH, bicarbonate, potassium, and lipid panel. 1, 2, 3
Core Kidney Function Assessment
The foundation of CKD 3b monitoring requires:
Serum creatinine with calculated eGFR - This establishes your baseline kidney function and should be measured at least annually, though more frequent monitoring (every 3-6 months) is appropriate at this stage given the higher risk of progression 1, 2
Urine albumin-to-creatinine ratio (ACR) on an early morning spot urine sample - This is the preferred method for proteinuria assessment over 24-hour collections or dipstick alone 1. If ACR ≥30 mg/g, confirm with a repeat early morning sample 1
Consider cystatin C measurement if you need confirmation of the eGFR or suspect the creatinine-based estimate may be inaccurate due to extremes of muscle mass or dietary factors 1, 2
Mineral and Bone Metabolism
At CKD stage 3b, mineral bone disease evaluation becomes critical:
Serum calcium, phosphorus, and intact PTH - These should be measured at baseline and monitored every 3-6 months in stage 3b 3. The target intact PTH range for stage 3b is 35-70 pg/mL 3
25-hydroxyvitamin D - Check at baseline and every 6-12 months once in target range 4
Anemia Workup
Anemia becomes increasingly prevalent at this stage:
Complete blood count (CBC) with hemoglobin, white blood cells, and platelets 5
Iron studies: serum ferritin and transferrin saturation (TSAT) 5. In non-dialysis CKD patients, ferritin <25 ng/mL in males or <11 ng/mL in females suggests iron deficiency 5
Reticulocyte count to assess bone marrow response 5
Vitamin B12 and folate if mean corpuscular volume is low 5
Metabolic and Cardiovascular Risk Assessment
Serum bicarbonate - Metabolic acidosis (bicarbonate <22 mmol/L) is common in CKD 3b and requires treatment 6
Serum potassium - Essential baseline given the risk of hyperkalemia with RAS inhibitors and other CKD therapies 6
Fasting lipid panel (total cholesterol, LDL, HDL, triglycerides) - All patients with CKD 3b should be on statin therapy for cardiovascular protection 2
Hemoglobin A1c if diabetic 2
Additional Considerations
Urinalysis with microscopy - Beyond ACR, examine for hematuria, casts, or other abnormalities that might suggest glomerulonephritis or other specific kidney diseases 1, 5
Serum albumin - Useful for interpreting calcium levels (use corrected calcium) and assessing nutritional status 3
Common Pitfalls to Avoid
Do not rely on serum creatinine alone - Always calculate and report eGFR, as creatinine can be misleading in patients with low muscle mass 1, 2. The 2021 CKD-EPI equation without race is now recommended 2.
Do not use dipstick urinalysis alone for proteinuria - Quantitative ACR is superior and required for risk stratification 1. The term "microalbuminuria" should no longer be used 1.
Do not delay PTH measurement - By CKD 3b, secondary hyperparathyroidism is common and requires early intervention to prevent bone disease 3.
Do not assume normal hemoglobin rules out anemia - Use gender-specific thresholds: <13.0 g/dL in males and <12.0 g/dL in females 5.
Monitoring Frequency
After establishing baseline values, monitor eGFR and ACR at least annually, but every 3-6 months is more appropriate for CKD 3b given the higher progression risk 1. Mineral metabolism labs should be checked every 3-6 months 3, while hemoglobin can be monitored every 3-12 months depending on whether anemia is present and being treated 5.
This comprehensive baseline assessment allows you to identify complications early, initiate nephroprotective therapies (ACE inhibitors/ARBs, SGLT2 inhibitors, statins), and determine appropriate timing for nephrology referral, which should be considered when eGFR approaches 30 ml/min/1.73 m² 2, 7.