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

Which group of findings fits hypercalcemia?

  1. ATetany, muscle cramps and a positive Chvostek’s sign
  2. BConstipation, fatigue, confusion and kidney stones
  3. CThirst with very dilute urine as the only signs
  4. DPeaked T waves and muscle weakness only
2.Lab values & electrolytesHard

A patient with severe sepsis is oozing blood from IV sites. Platelets 40 × 10³/µL, fibrinogen low, D-dimer very high, PT and aPTT both prolonged. Which condition fits best?

  1. ADisseminated intravascular coagulation (DIC)
  2. BHeparin-induced thrombocytopenia (HIT)
  3. CImmune thrombocytopenic purpura (ITP)
  4. DHemophilia A (factor VIII deficiency)
3.Arterial blood gas interpretationMedium

A practice gas shows a metabolic acidosis with HCO₃⁻ 12 mmol/L. Use Winter’s formula to find the expected PaCO₂ (the centre of the range) in mmHg.

HCO₃⁻ 12 mmol/L
PaCO₂ = mmHg
4.Arterial blood gas interpretationHard

Practice values: Na⁺ 136, Cl⁻ 107, HCO₃⁻ 14 mmol/L and albumin 37 g/L. Find the albumin-corrected anion gap AGc = AG + 0.25·(40 − albumin).

Na⁺ 136 mmol/LCl⁻ 107 mmol/LHCO₃⁻ 14 mmol/Lalbumin 37 g/L
AGc = mmol/L
5.Clinical pharmacokineticsMedium
110t (h)C (mg/L)C₀ = 6 mg/Lt½ = 3.5 hC₀/2

Practice exercise (made-up values, not clinical guidance). A patient receives an IV bolus of 110 mg of gentamicin. The level back-extrapolated to time zero is 6 mg/L and the half-life is 3.5 h. Find the clearance CL in L/h.

D = 110 mgC₀ = 6 mg/Lt½ = 3.5 h
CL = L/h
6.Clinical pharmacokineticsHard
Vd = 7 L/kg0.2 → 0.8 mcg/LLD = ? mcg87 kg · S = 1 · F = 0.7

Practice exercise (made-up values, not clinical guidance). An 87 kg patient has a digoxin level of 0.2 mcg/L; the target is 0.8 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?

87 kgVd = 7 L/kgC = 0.2 → 0.8 mcg/LF = 0.7
LD = mcg
7.Clinical nursingMedium

What should the nurse regularly check for a patient with a syringe driver?

  1. AOnly the patient's weight, measured once a week on the same scales
  2. BOnly that the battery is charged and the device is switched on
  3. CNothing, once the syringe driver is running
  4. DThe infusion site, the rate and volume remaining, and the patient's symptoms
8.Clinical nursingHard

A nurse realises she has given a patient a medicine prescribed for another patient. What should she do first?

  1. AAsk a colleague not to mention it
  2. BWait to see whether any side effects develop
  3. CGive the patient's own medicine as well to balance it
  4. DCheck the patient's condition and inform the doctor

3Solutions

1.Lab values & electrolytes

Answer: B · Constipation, fatigue, confusion and kidney stones

Why this is the answer

High calcium reduces nerve and muscle excitability, causing fatigue, constipation, confusion, bone pain and kidney stones. Tetany and Chvostek’s sign are signs of low calcium.Checked against: Calcium Blood Test · MedlinePlus (NIH) (2024)

Answer

B · Constipation, fatigue, confusion and kidney stones
2.Lab values & electrolytes

Answer: A · Disseminated intravascular coagulation (DIC)

Why this is the answer

In DIC, widespread clotting consumes platelets and clotting factors, so PT and aPTT are prolonged and fibrinogen falls, while clot breakdown raises the D-dimer. ITP lowers platelets only, and hemophilia A prolongs only the aPTT.Checked against: Fibrinogen blood test · MedlinePlus (NIH) (2025); Prothrombin time (PT) · MedlinePlus (NIH) (2025); D-Dimer Test · MedlinePlus (NIH) (2024)

Answer

A · Disseminated intravascular coagulation (DIC)
3.Arterial blood gas interpretation

Answer: PaCO₂ = 26 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 = 1.5·12 + 8 = 26 mmHg
range: 24–28 mmHg

Answer

PaCO₂ = 26 mmHg
4.Arterial blood gas interpretation

Answer: AGc = 15.75 mmol/L

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.

Anion gap

AG = 136 − (107 + 14) = 15

Correction for albumin

15 + 0.25·(40 − 37) = 15.75 mmol/L

Answer

AGc = 15.75 mmol/L

Above about 12 after correction: a high anion gap.

5.Clinical pharmacokinetics

Answer: CL = 3.631 L/h

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 = 110/6 = 18.33 L

Rate constant

k = 0.693/3.5 = 0.198 h⁻¹

Clearance

CL = 0.198·18.33 = 3.631 L/h

Answer

CL = 3.631 L/h
6.Clinical pharmacokinetics

Answer: LD = 522 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 = 7·87 = 609 L

Rise needed

0.8 − 0.2 = 0.6 mcg/L

Loading dose

LD = 609·0.6/(1·0.7) = 522 mcg

Answer

LD = 522 mcg

In practice the dose is rounded to a convenient amount.

7.Clinical nursing

Answer: D · The infusion site, the rate and volume remaining, and the patient's symptoms

Why this is the answer

Nurses check that the infusion is running as planned, the site has no redness or swelling, and the symptoms are controlled. Problems are acted on quickly to avoid distress.

Answer

D · The infusion site, the rate and volume remaining, and the patient's symptoms
8.Clinical nursing

Answer: D · Check the patient's condition and inform the doctor

Why this is the answer

The first priority is the patient: assess them and inform the prescriber so that any treatment can be given. The error must then be documented, reported and discussed openly with the patient (open disclosure; in the UK, the statutory duty of candour).

Answer

D · Check the patient's condition and inform the doctor

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