Normal Laboratory Values for Coagulation and Electrolyte Parameters
The normal reference ranges for adults are: PT 11-13.5 seconds, aPTT 25-35 seconds, INR 1.0 (with therapeutic range 2.0-3.0 for most anticoagulation indications), serum potassium 3.5-5.0 mEq/L, and serum magnesium 1.7-2.2 mg/dL. 1
Coagulation Parameters
Prothrombin Time (PT)
- Normal range: 11-13.5 seconds 1
- PT ratio should be <1.4 times control 1
- PT measures the extrinsic and common pathways of coagulation 1
- Values may vary slightly between laboratories depending on reagent and instrument sensitivity 2, 3
Activated Partial Thromboplastin Time (aPTT)
- Normal range: 25-35 seconds 1
- aPTT ratio should be <1.4 times control 1
- aPTT measures the intrinsic and common pathways of coagulation 1
- Significant variability exists between different commercial aPTT tests and instruments 4, 3
International Normalized Ratio (INR)
- Normal baseline: 1.0-1.3 1
- Therapeutic range for warfarin: 2.0-3.0 for most indications (atrial fibrillation, DVT, PE) 1, 5
- Higher therapeutic range: 2.5-3.5 for mechanical prosthetic heart valves 5
- An INR of 1.4 represents minimal elevation and falls within acceptable laboratory variation 1
Electrolyte Parameters
Serum Potassium (K)
- Normal range: 3.5-5.0 mEq/L
- Critical low: <2.5 mEq/L
- Critical high: >6.5 mEq/L
Serum Magnesium (Mg)
- Normal range: 1.7-2.2 mg/dL (or 1.4-1.8 mEq/L)
- Critical low: <1.2 mg/dL
- Critical high: >4.9 mg/dL
Clinical Significance for Procedures
When PT/aPTT Ratios Are Concerning
- PT ratio or aPTT ratio >1.4 times normal control is generally considered a relative contraindication to invasive procedures 1
- For emergency surgery or life-threatening hemorrhage, maintain PT/aPTT <1.5 times normal control 5
- Platelet count should also be evaluated, with counts <100,000/mm³ considered a relative contraindication 1
Important Caveats About INR
- INR was specifically designed and validated only for monitoring vitamin K antagonist therapy, not as a general predictor of bleeding risk in other contexts 1
- In patients not on warfarin, INR has limited predictive value for bleeding complications 1
- Normal PT/aPTT values don't exclude all bleeding disorders, particularly those affecting platelet function 1
Laboratory Variability Considerations
Instrument and Reagent Effects
- Different coagulation instruments may yield different PT and INR results even after correcting for the International Sensitivity Index (ISI) 2
- Each laboratory must establish its own reference ranges based on reagent and instrument sensitivity 3
- Whole blood point-of-care testing shows good correlation with laboratory PT values but may show discordance with aPTT measurements 4