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.

New problemsBack to the course

In the print window, choose “Save as PDF” to keep a copy on your device.

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

In chronic kidney disease, a high serum phosphate is usually accompanied by which change in calcium?

  1. AA low serum calcium
  2. BA high serum calcium
  3. CA high serum potassium only, with no calcium change
  4. DNo change in calcium
2.Lab values & electrolytesHard

After chemotherapy for a fast-growing lymphoma, which pattern of results suggests tumor lysis syndrome?

  1. AHigh sodium and low glucose only
  2. BNormal electrolytes with a high hemoglobin
  3. CHigh potassium, high phosphate, high uric acid and low calcium
  4. DLow potassium, low phosphate, low uric acid and a raised calcium
3.Arterial blood gas interpretationMedium

A practice gas shows a metabolic acidosis with HCO₃⁻ 16 mmol/L. Use Winter’s formula to find the expected PaCO₂ (the centre of the range) in mmHg.

HCO₃⁻ 16 mmol/L
PaCO₂ = mmHg
4.Arterial blood gas interpretationHard

A practice gas shows a metabolic alkalosis with HCO₃⁻ 40 mmol/L. Use expected PaCO₂ = 0.7·HCO₃⁻ + 21 to find the expected PaCO₂.

HCO₃⁻ 40 mmol/L
PaCO₂ = mmHg
5.Clinical pharmacokineticsMedium
Vd = 0.5 L/kgC = 15 mg/LLD = ? mg67 kg · S = 0.8 · F = 1

Practice exercise (made-up values, not clinical guidance). A 67 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.

67 kgVd = 0.5 L/kgC = 15 mg/LS = 0.8
LD = mg
6.Clinical pharmacokineticsHard
Vd = 0.3 L/kg0.5 → 8 mg/LD = ? mg52 kg

Practice exercise (made-up values, not clinical guidance). A 52 kg patient on regular tobramycin doses is at steady state. The target is a peak of 8 mg/L and a trough of 0.5 mg/L, and the volume of distribution is 0.3 L/kg. Assuming rapid IV doses, what dose D raises the level from the trough to the peak?

52 kgVd = 0.3 L/kgCpeak = 8 mg/LCtrough = 0.5 mg/L
D = mg
7.Clinical nursingMedium

Which pattern is typical of relapsing–remitting multiple sclerosis?

  1. ASudden severe headache with neck stiffness
  2. BConstant weakness that starts in the feet and spreads upwards after a viral illness
  3. CA single stroke that leaves permanent weakness
  4. DAttacks of symptoms, such as visual loss or numbness, that improve between relapses
8.Clinical nursingHard

A nurse teaches a patient with hypertension about their medicines. Which statement shows a need for further teaching?

  1. A"I'll have my blood pressure checked regularly."
  2. B"I'll keep taking my tablets even if I feel well."
  3. C"I'll stop my tablets once my readings are normal."
  4. D"I'll get up slowly from sitting."

3Solutions

1.Lab values & electrolytes

Answer: A · A low serum calcium

Why this is the answer

When failing kidneys retain phosphate, phosphate binds calcium and vitamin D activation falls, so serum calcium tends to drop. This drives secondary hyperparathyroidism and bone disease.Checked against: Hypocalcemia (StatPearls) · StatPearls (NCBI Bookshelf) (2026); Phosphate in Blood · MedlinePlus (NIH) (2023)

Answer

A · A low serum calcium
2.Lab values & electrolytes

Answer: C · High potassium, high phosphate, high uric acid and low calcium

Why this is the answer

When many tumor cells break down, potassium, phosphate and nucleic acids (which become uric acid) flood the blood. Phosphate binds calcium, so calcium falls. The risks include arrhythmias and acute kidney injury.Checked against: Tumor Lysis Syndrome (StatPearls) · StatPearls (NCBI Bookshelf) (2024)

Answer

C · High potassium, high phosphate, high uric acid and low calcium
3.Arterial blood gas interpretation

Answer: PaCO₂ = 32 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 = 1.5·16 + 8 = 32 mmHg
range: 30–34 mmHg

Answer

PaCO₂ = 32 mmHg
4.Arterial blood gas interpretation

Answer: PaCO₂ = 49 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.

Expected PaCO₂

0.7·40 + 21 = 49 mmHg

Answer

PaCO₂ = 49 mmHg

The rise has a limit: PaCO₂ rarely goes above about 55 mmHg from compensation alone.

5.Clinical pharmacokinetics

Answer: LD = 628.1 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 = 0.5·67 = 33.5 L

Amount of active drug needed

Vd·C = 33.5·15 = 502.5 mg

Loading dose

LD = 502.5/(0.8·1) = 628.1 mg

Answer

LD = 628.1 mg

In practice the dose is rounded to a convenient amount.

6.Clinical pharmacokinetics

Answer: D = 117 mg

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 = 0.3·52 = 15.6 L

Dose

D = 15.6·(8 − 0.5) = 117 mg

Answer

D = 117 mg

In practice the dose is rounded to a convenient amount.

7.Clinical nursing

Answer: D · Attacks of symptoms, such as visual loss or numbness, that improve between relapses

Why this is the answer

MS causes demyelination in the brain and spinal cord. Many patients first have attacks such as optic neuritis or sensory symptoms, with partial or full recovery between relapses.

Answer

D · Attacks of symptoms, such as visual loss or numbness, that improve between relapses
8.Clinical nursing

Answer: C · "I'll stop my tablets once my readings are normal."

Why this is the answer

Hypertension usually needs long-term treatment; stopping because readings are normal often makes blood pressure rise again. Patients should discuss any changes with their prescriber.

Answer

C · "I'll stop my tablets once my readings are normal."

ExamProblems · examproblems.com/courses/lab-values-for-nurses