What are blood, blood type, and clinical chemistry basics?
This page groups seven small calculators that share a common thread: blood. Blood is a suspension of red cells, white cells, and platelets in a watery liquid called plasma; if you let the cells clot and spin down the supernatant, you get serum (plasma minus fibrinogen and the other clotting proteins). Most clinical chemistry panels — electrolytes, glucose, BUN, calcium, liver enzymes — are measured on serum, while blood typing and BAC are measured on whole blood or plasma.
Three vocab words to keep straight: Antigens are markers on the outside of red cells (the A antigen, the B antigen, the D antigen of Rh). Antibodies are proteins in the plasma that bind and destroy cells with the wrong antigens — a type O person carries anti-A and anti-B, a type A person carries only anti-B. Plasma is unclotted, serum is what remains after clotting, and the difference is the clotting factors, which is why coagulation tests are done on plasma but chemistry panels on serum.
Blood type inheritance — ABO and Rh
The ABO system is governed by three alleles of one gene: A and B are codominant (both expressed when present), O is recessive and silent. Phenotype A can therefore be genotype AA or AO, phenotype B can be BB or BO, phenotype AB is always AB, phenotype O is always OO. Rh is much simpler: a single gene with two alleles, D (Rh+) dominant over d (Rh−). A Rh+ person is either DD or Dd; Rh− is always dd. The cross of two Rh+ parents can produce Rh− only if both are Dd, giving 25% dd offspring. This page combines ABO and Rh independently (Mendel's law of segregation for each locus). With the sample (A+ × B+), ABO yields AB 56.25% / A 18.75% / B 18.75% / O 6.25%; this page treats each Rh+ parent as a known Dd carrier, giving Rh+ 75% / Rh− 25%. Multiplying gives AB+ 42.2%, AB− 14.1%, A+ 14.1%, B+ 14.1%, A− 4.7%, B− 4.7%, O+ 4.7%, O− 1.6%.
Blood alcohol — Widmark formula
Ethanol distributes throughout the body's water, so blood alcohol concentration depends on how much water the drinker carries. The Widmark equation BAC(%) = (grams × 100) / (weight_g × r) − β × t captures this with a single factor r: 0.68 for adult males (about 68% of body mass is water) and 0.55 for adult females (women carry proportionally less water because of higher adipose content). The second term, β × t, is the liver's first-order metabolism at about 0.015 g/dL per hour — about one standard drink per hour. Sample (70 kg male, 3 drinks × 14 g ethanol, 2 hours elapsed) gives BACraw = 42×100 / (70000×0.68) = 0.0882%, then 0.0882 − 0.015×2 = 0.058% (58 mg/dL) — still legally impaired in most jurisdictions. The formula is intentionally simple: it ignores food (which slows absorption), habitual drinking (which induces CYP2E1), genetics (ADH and ALDH polymorphisms common in East Asian populations), and individual water fraction. Use the result as an order-of-magnitude estimate, not a court-of-law number. At the average elimination rate, a 0.08% BAC needs about 5.3 hours to reach zero, and absorption adds another 30–90 minutes — so "when is it safe to drive" usually lands six-plus hours after the last drink.
Blood pressure classification
Blood pressure is reported as systolic over diastolic — the peak during ventricular contraction over the trough during ventricular relaxation. The 2017 ACC/AHA guideline redefined hypertension at lower thresholds than the older JNC-7: Normal <120/<80, Elevated 120–129/<80, Stage 1 130–139 or 80–89, Stage 2 ≥140 or ≥90, Hypertensive crisis >180/>120. China's 2024 guideline keeps the older 140/90 cutoff but adds 3-grade severity (轻度 / 中度 / 重度) once hypertension is diagnosed. Sample (135/85) classifies as ACC/AHA Stage 1 Hypertension and Chinese Pre-hypertension (正常高值).
Anion gap — the acid-base accounting trick
Serum is electrically neutral overall, so the sum of measured cations must equal the sum of measured anions plus unmeasured anions. We measure the major cation (Na⁺, sometimes with K⁺) and the major anions (Cl⁻ and HCO₃⁻), then AG = Na⁺ − Cl⁻ − HCO₃⁻; the missing amount is "unmeasured anions" — proteins, sulfate, phosphate, organic acids. Normal is 8–12 mEq/L without K (12–16 with K). A high gap (AG > 12) suggests an unmeasured anion is accumulating: lactate in lactic acidosis, β-hydroxybutyrate in diabetic ketoacidosis, ingested toxins like methanol (formate), ethylene glycol (oxalate), or salicylate. Sample (Na=140, K=4.0, Cl=105, HCO₃=24) gives AG without K = 11 mEq/L (normal), AG with K = 15 mEq/L (normal) — consistent with no unmeasured anion accumulation.
Plasma osmolality — counting dissolved particles
Serum osmolality counts the total number of dissolved solute particles per kilogram of water, in units of mOsm/kg. Sodium and its accompanying anions dominate (the formula uses 2×Na to count both the Na⁺ cation and its partner Cl⁻/HCO₃⁻ anion), with glucose and urea contributing the next-largest amounts. The bedside estimate Osm = 2×Na + Glu/18 + BUN/2.8 bakes in the mg/dL→mmol/L conversions for glucose (MW 180) and urea (MW 28). Normal is 285–295 mOsm/kg. The osmolar gap (measured − calculated) flags low-molecular-weight toxins like ethylene glycol, methanol, isopropanol, and mannitol that the standard chemistry panel doesn't measure. Sample (Na=140, Glu=90, BUN=14) gives 2×140 + 5.0 + 5.0 = 290 mOsm/kg — squarely normal.
Corrected calcium — adjusting for low albumin
About 40% of serum calcium is bound to albumin (and a smaller fraction to other proteins); only the free ionized fraction is biologically active. When albumin drops, total calcium drops too — even if ionized calcium is perfectly normal. The classic Payne correction adds 0.8 mg/dL for every 1 g/dL that albumin falls below the reference of 4.0: Corr Ca = Total Ca + 0.8 × (4.0 − Alb). Sample (Ca=8.0, Alb=3.0, AlbNorm=4.0) gives 8.0 + 0.8×1.0 = 8.8 mg/dL, comfortably in the 8.5–10.5 normal range. The 0.8 factor is an empirical average; in severe hypoalbuminemia or with abnormal binding (citrate from blood transfusion, paraproteinemia) the correction drifts and measuring ionized calcium directly becomes necessary.
Corrected sodium — pulling out the glucose effect
Hyperglycemia pulls water out of cells by osmosis, diluting the measured serum sodium and producing pseudohyponatremia: the patient's sodium is fine, the reading is artificially low. To recover the "true" sodium, add back the dilution: Corr Na = Na + 1.6 × (Glu − 100) / 100 (the 1.6 mEq per 100 mg/dL rule, also known as the Katz or Hillier rule). Sample (Na=125, Glu=500) gives 125 + 1.6×(400/100) = 125 + 6.4 = 131.4 mEq/L — still mildly low but closer to the 135–145 normal range. The 1.6 factor is for glucose in mg/dL; if the lab reports glucose in mmol/L, the equivalent rule is +2.4 per 5.6 mmol/L above normal (page documents both). Once glucose is corrected (with insulin and fluids), the measured sodium will rise back toward this corrected value.
Common misconceptions
- Type O is not the universal donor for plasma. Type O whole blood lacks A and B antigens (safe to give most recipients for red cells), but type O plasma carries anti-A and anti-B antibodies, which will attack the red cells of an A, B, or AB recipient. For plasma transfusion, type AB is the universal donor.
- Rh+ × Rh− cannot give Rh− unless both are heterozygous. Two Rh+ parents can have an Rh− baby only if both are
Dd (probability 1/4 per Rh− child). If either parent is DD, all children will be Rh+.
- The Widmark formula does not predict legal intoxication. Individual variation in water fraction, food in the stomach, drinking pace, liver enzyme induction, and time-to-absorption means two people drinking the same amount can have BAC values that differ by a factor of two or more. The number here is a population-average estimate, not a forensic measurement.
- An elevated anion gap is not a diagnosis. AG is a clue — it tells you that an unmeasured anion is present, not which one. You still need lactate, ketones, tox screen, and clinical context.
Related tools: Clinical Medicine for creatinine clearance and dosing, Body Metrics for BMI and body-fat estimation, and Nutrition & Diet for the energy and macronutrient context that determines total body water and Widmark's r factor.
