Lab Values for Nurses: cram pack

Formula sheets, the key method steps for every topic and 8 practice problems, with full solutions at the back.

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1Formulas and methods

Lab values & electrolytes

Method

  1. Compare the value with the laboratory’s own reference range, then with the patient’s baseline and trend.
  2. Check the sample: hemolysis, a delay, or blood drawn near an IV can give false results.
  3. Link the value to its typical signs and to the drugs and conditions that cause it.
  4. Look for interactions: low potassium or magnesium increases digoxin toxicity; dehydration raises lithium.
  5. Act on the patient: assess, hold a drug if the level is unsafe, read back critical values and inform the prescriber.

Arterial blood gas interpretation

Normal ranges
pH 7.35–7.45, PaCO₂ 35–45 mmHg (4.7–6.0 kPa), HCO₃⁻ 22–26 mmol/L
ROME
Respiratory: PaCO₂ opposite to pH; Metabolic: HCO₃⁻ equal to pH
Winter’s formula
PaCO₂ = 1.5·HCO₃⁻ + 8 ± 2
Metabolic alkalosis
PaCO₂ ≈ 0.7·HCO₃⁻ + 21
Anion gap
AG = Na⁺ − (Cl⁻ + HCO₃⁻) ≈ 8–12
Albumin-corrected gap
AG + 2.5·(4.0 − alb g/dL) = AG + 0.25·(40 − alb g/L)
Delta ratio
(AG − 12)/(24 − HCO₃⁻)
P/F ratio
PaO₂/FiO₂
Alveolar gas equation
PAO₂ = FiO₂·(760 − 47) − PaCO₂/0.8
Serum osmolality
2·Na⁺ + glucose + urea (2·Na⁺ + glucose/18 + BUN/2.8)

Method

  1. Read the pH: below 7.35 is acidosis, above 7.45 alkalosis.
  2. Find the value that explains it: PaCO₂ moving opposite to the pH is respiratory, HCO₃⁻ moving with the pH is metabolic.
  3. Look at the other value: normal means uncompensated, abnormal in the compensating direction means partial compensation, and a normal pH with both abnormal means full compensation.
  4. In a metabolic acidosis, compare PaCO₂ with Winter’s formula, 1.5·HCO₃⁻ + 8 ± 2, and calculate the anion gap Na⁺ − (Cl⁻ + HCO₃⁻).
  5. Check a high anion gap with the delta ratio and, when albumin is low, with the corrected gap.

Clinical pharmacokinetics

Rate constant and half-life
k = ln 2/t½ = 0.693/t½
First-order decay
C = C₀·e−kt = C₀·(1/2)t/t½
Two levels
k = ln(C₁/C₂)/Δt
Volume and clearance
Vd = dose/C₀, CL = k·Vd
Loading dose
LD = Vd·C/(S·F)
Maintenance rate
R = CL·Css/(S·F)
Average steady state
Css,avg = S·F·D/(CL·τ)
Steady state
1 − (1/2)ⁿ after n half-lives; ≈ 97% after 5
Infusion
C = (R/CL)(1 − e−kt)
Interval and dose from peak and trough
τ = ln(Cpeak/Ctrough)/k, D = Vd·(Cpeak − Ctrough)
Cockcroft–Gault
CrCl = (140 − age) × kg/(72 × SCr mg/dL), × 0.85 for women
Renal adjustment
dose × CrCl/normal; Dettli Q = 1 − fe·(1 − CrCl/normal)

Method

  1. Convert the half-life to a rate constant: k = 0.693/t½.
  2. Turn per-kg values into totals: Vd (L) = L/kg × weight, CL (L/h) = L/kg/h × weight.
  3. Pick the relation: C = C₀·e−kt, LD = Vd·C/(S·F), maintenance rate = CL·Css/(S·F), τ = ln(Cpeak/Ctrough)/k.
  4. For kidney function, estimate CrCl with Cockcroft–Gault first, then scale the dose or the interval.
  5. Check the result: levels fall by half every half-life, and steady state takes about five half-lives.

2Practice problems

1.Lab values & electrolytesMedium

A patient on warfarin started eating large daily portions of kale and spinach. The INR has fallen. What advice is most appropriate?

  1. ADouble the warfarin dose without telling the prescriber
  2. BNever eat green vegetables again
  3. CStop warfarin until the diet is normal
  4. DKeep the intake of vitamin K–rich foods steady from day to day
2.Lab values & electrolytesHard

A patient has a hemoglobin of 11.2 g/dL (112 g/L), an MCV of 64 fL, a normal ferritin and a normal-to-high red cell count. What is the most likely cause?

  1. AVitamin B12 deficiency
  2. BIron deficiency
  3. CThalassemia trait
  4. DAcute blood loss
3.Arterial blood gas interpretationMedium

A practice gas shows a metabolic acidosis with HCO₃⁻ 18 mmol/L. Use Winter’s formula to find the expected PaCO₂ (the centre of the range) in mmHg.

HCO₃⁻ 18 mmol/L
PaCO₂ = mmHg
4.Arterial blood gas interpretationHard
pH7.357.457.21PaCO₂mmHg354518HCO₃⁻mmol/L22267appropriate compensationadded respiratory acidosisadded respiratory alkalosis

A practice gas shows a metabolic acidosis: pH 7.21, PaCO₂ 18 mmHg (2.4 kPa), HCO₃⁻ 7 mmol/L. Compare the measured PaCO₂ with Winter’s formula. Click your interpretation.

pH 7.21PaCO₂ 18 mmHg (2.4 kPa)HCO₃⁻ 7 mmol/L
  1. 1appropriate compensation
  2. 2added respiratory acidosis
  3. 3added respiratory alkalosis
5.Clinical pharmacokineticsMedium
Vd = 0.5 L/kgC = 15 mg/LLD = ? mg75 kg · S = 0.8 · F = 1

Practice exercise (made-up values, not clinical guidance). A 75 kg patient is to be loaded intravenously with aminophylline to reach a theophylline level of 15 mg/L. The volume of distribution is 0.5 L/kg and the salt factor is S = 0.8. Find the loading dose LD of aminophylline in mg.

75 kgVd = 0.5 L/kgC = 15 mg/LS = 0.8
LD = mg
6.Clinical pharmacokineticsHard
Vd = 7 L/kg0.4 → 1 mcg/LLD = ? mcg71 kg · S = 1 · F = 0.7

Practice exercise (made-up values, not clinical guidance). A 71 kg patient has a digoxin level of 0.4 mcg/L; the target is 1 mcg/L. The volume of distribution is 7 L/kg and the tablets have F = 0.7. What extra loading dose LD raises the level to the target?

71 kgVd = 7 L/kgC = 0.4 → 1 mcg/LF = 0.7
LD = mcg
7.Clinical nursingMedium

A patient with COPD and known carbon dioxide retention needs a nebulised bronchodilator. How should it usually be given?

  1. ADriven by high-flow oxygen at 15 L/min for as long as possible
  2. BWithout monitoring oxygen saturation
  3. CDriven by compressed air, with oxygen given separately if needed
  4. DOnly with the patient lying flat
8.Clinical nursingHard

A nurse is unsure whether a prescribed dose of a high-risk drug is correct. What is the safest action?

  1. AWithhold it and check with the prescriber or pharmacist first
  2. BGive the dose because the doctor has signed the prescription
  3. CAsk the patient whether the dose looks the same as usual and give it
  4. DGive half the dose as a compromise until the doctor is available

3Solutions

1.Lab values & electrolytes

Answer: D · Keep the intake of vitamin K–rich foods steady from day to day

Why this is the answer

Vitamin K counteracts warfarin, so a large increase in leafy greens lowers the INR. Patients do not have to avoid these foods, but they should keep the amount consistent so the dose can be set correctly.Checked against: Warfarin · MedlinePlus (NIH) (2026); Taking warfarin · MedlinePlus (NIH) (2025)

Answer

D · Keep the intake of vitamin K–rich foods steady from day to day
2.Lab values & electrolytes

Answer: C · Thalassemia trait

Why this is the answer

Thalassemia trait causes microcytosis from reduced globin production, often with a normal or high red cell count and normal ferritin. Iron deficiency has low ferritin. Iron supplements do not help thalassemia trait and can cause iron overload.Checked against: Thalassemia - Medical Encyclopedia · MedlinePlus (NIH) (2026); Red Blood Cell (RBC) Indices · MedlinePlus (NIH) (2024)

Answer

C · Thalassemia trait
3.Arterial blood gas interpretation

Answer: PaCO₂ = 35 mmHg

Expected compensation: in metabolic acidosis Winter’s formula predicts PaCO₂ = 1.5·HCO₃⁻ + 8 ± 2 mmHg; a measured value above that range points to an additional respiratory acidosis, below it to an additional respiratory alkalosis. The anion gap AG = Na⁺ − (Cl⁻ + HCO₃⁻), normally about 8–12 mmol/L, separates high-gap from normal-gap metabolic acidosis; low albumin hides a high gap (add 2.5 per g/dL that albumin is below 4.0). The delta ratio (AG − 12)/(24 − HCO₃⁻) between about 0.8 and 2 fits a pure high-gap acidosis (some texts use 1 instead of 0.8), below that an added normal-gap acidosis, and above 2 an added metabolic alkalosis.

Winter’s formula

PaCO₂ = 1.5·18 + 8 = 35 mmHg
range: 33–37 mmHg

Answer

PaCO₂ = 35 mmHg
4.Arterial blood gas interpretation

Answer: appropriate compensation

Expected compensation: in metabolic acidosis Winter’s formula predicts PaCO₂ = 1.5·HCO₃⁻ + 8 ± 2 mmHg; a measured value above that range points to an additional respiratory acidosis, below it to an additional respiratory alkalosis. The anion gap AG = Na⁺ − (Cl⁻ + HCO₃⁻), normally about 8–12 mmol/L, separates high-gap from normal-gap metabolic acidosis; low albumin hides a high gap (add 2.5 per g/dL that albumin is below 4.0). The delta ratio (AG − 12)/(24 − HCO₃⁻) between about 0.8 and 2 fits a pure high-gap acidosis (some texts use 1 instead of 0.8), below that an added normal-gap acidosis, and above 2 an added metabolic alkalosis.

Expected PaCO₂

1.5·7 + 8 = 18.5 → 16.5–20.5 mmHg

Compare

measured: 18 mmHg → appropriate compensation

Answer

appropriate compensation
5.Clinical pharmacokinetics

Answer: LD = 703.1 mg

Clearance and volume: at steady state the dosing rate equals the elimination rate, so the maintenance rate = CL × Css. A loading dose is LD = Vd × C/(S × F), with S the salt factor (fraction of active drug in the salt) and F the bioavailability (1 for IV). From two levels, k = ln(C₁/C₂)/Δt; for an IV bolus Vd = dose/C₀ and CL = k × Vd. The numbers are for practice only; at the bedside always follow local protocols and have high-risk doses double-checked.

Volume of distribution

Vd = 0.5·75 = 37.5 L

Amount of active drug needed

Vd·C = 37.5·15 = 562.5 mg

Loading dose

LD = 562.5/(0.8·1) = 703.1 mg

Answer

LD = 703.1 mg

In practice the dose is rounded to a convenient amount.

6.Clinical pharmacokinetics

Answer: LD = 426 mcg

Designing a regimen: with rapid IV doses the level falls from peak to trough during the interval, so τ = ln(Cpeak/Ctrough)/k and the dose that restores the peak is Vd × (Cpeak − Ctrough). The average steady-state level is Css,avg = S·F·dose/(CL·τ). An infusion rises as C = (R/CL)(1 − e−kt). For a drug cleared by the kidneys, the dose is scaled with the creatinine clearance. The numbers are for practice only; at the bedside always follow local protocols and have high-risk doses double-checked.

Volume of distribution

Vd = 7·71 = 497 L

Rise needed

1 − 0.4 = 0.6 mcg/L

Loading dose

LD = 497·0.6/(1·0.7) = 426 mcg

Answer

LD = 426 mcg

In practice the dose is rounded to a convenient amount.

7.Clinical nursing

Answer: C · Driven by compressed air, with oxygen given separately if needed

Why this is the answer

Nebulisers driven by high-flow oxygen can raise PaCO2 in patients at risk. Air-driven nebulisers are used, with supplementary oxygen via nasal cannulae to keep saturation in the target range.

Answer

C · Driven by compressed air, with oxygen given separately if needed
8.Clinical nursing

Answer: A · Withhold it and check with the prescriber or pharmacist first

Why this is the answer

Nurses must understand any drug they administer. If a dose seems wrong, they should check with the prescriber or pharmacist and document the action.

Answer

A · Withhold it and check with the prescriber or pharmacist first

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