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

In the US, when is a pregnant woman without known risk factors usually screened for gestational diabetes?

  1. AAt about 6–8 weeks of pregnancy
  2. BAt about 38–40 weeks of pregnancy
  3. COnly after the baby is born
  4. DAt about 24–28 weeks of pregnancy
2.Lab values & electrolytesHard

Which patient should the nurse assess first?

  1. AChest pain with troponin rising from the first to the second sample
  2. BHeart failure with a BNP lower than the result on admission
  3. CHypertension with a potassium of 3.6 mEq/L on a thiazide
  4. DStable angina with a total cholesterol of 245 mg/dL (6.3 mmol/L)
3.Arterial blood gas interpretationMedium

A practice gas shows a metabolic acidosis with HCO₃⁻ 8 mmol/L. Use Winter’s formula to find the expected PaCO₂ (the centre of the range) in mmHg.

HCO₃⁻ 8 mmol/L
PaCO₂ = mmHg
4.Arterial blood gas interpretationHard

Practice values: Na⁺ 139, Cl⁻ 107, HCO₃⁻ 22 mmol/L and albumin 37 g/L. Find the albumin-corrected anion gap AGc = AG + 0.25·(40 − albumin).

Na⁺ 139 mmol/LCl⁻ 107 mmol/LHCO₃⁻ 22 mmol/Lalbumin 37 g/L
AGc = mmol/L
5.Clinical pharmacokineticsMedium
110t (h)C (mg/L)1 h6 mg/L6 h1.5 mg/L

Practice exercise (made-up values, not clinical guidance). After an IV dose of tobramycin, the level is 6 mg/L at 1 h and 1.5 mg/L at 6 h. Assume first-order elimination. Find the half-life t½.

C1 = 6 mg/L (1 h)C2 = 1.5 mg/L (6 h)
t½ = h
6.Clinical pharmacokineticsHard
90 kg72 · womanSCr 1.8 mg/dLCrCl → D = ? mg

Practice exercise (made-up values, not clinical guidance). A 72-year-old woman weighs 90 kg and has a serum creatinine of 1.8 mg/dL (159 µmol/L). Estimate the creatinine clearance with Cockcroft–Gault: CrCl (mL/min) = (140 − age) × weight (kg)/(72 × SCr in mg/dL), × 0.85 for women. The usual cefazolin dose is 1000 mg every 8 h for a creatinine clearance of 120 mL/min. Keeping the interval, the dose is reduced in proportion to the creatinine clearance. Find the adjusted dose D. Round to the nearest whole number.

72 years90 kgwomanSCr = 1.8 mg/dL1000 mg q8hCrCl normal 120 mL/min
D = mg
7.Clinical nursingMedium

Which signs suggest a blood transfusion reaction in the first minutes?

  1. ASlow, steady rise in haemoglobin over the next day
  2. BImproved colour and energy
  3. CFever, chills, back pain and low blood pressure
  4. DMild hunger
8.Clinical nursingHard

A nurse notices that a wrong dose of a drug was prepared but was caught before it was given. What should be done?

  1. AReport it as a near miss through the incident reporting system
  2. BDiscipline the nurse who prepared it on the spot in front of colleagues
  3. CSay nothing, because no harm reached the patient
  4. DTell only the patient's family

3Solutions

1.Lab values & electrolytes

Answer: D · At about 24–28 weeks of pregnancy

Why this is the answer

Placental hormones increase insulin resistance in the second half of pregnancy, so US guidance recommends screening all women at 24–28 weeks, with earlier testing for those with risk factors. In the UK, NICE recommends an oral glucose tolerance test at 24–28 weeks only for women with risk factors.Checked against: Glucose screening tests during pregnancy · MedlinePlus (NIH) (2026); Gestational diabetes · MedlinePlus (NIH) (2026)

Answer

D · At about 24–28 weeks of pregnancy
2.Lab values & electrolytes

Answer: A · Chest pain with troponin rising from the first to the second sample

Why this is the answer

A rising troponin with chest pain indicates acute myocardial injury, possibly infarction, which needs urgent assessment and treatment. High cholesterol is a long-term risk, a falling BNP shows improvement, and a potassium of 3.6 mEq/L is normal.Checked against: Troponin Test · MedlinePlus (NIH) (2023)

Answer

A · Chest pain with troponin rising from the first to the second sample
3.Arterial blood gas interpretation

Answer: PaCO₂ = 20 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 = 1.5·8 + 8 = 20 mmHg
range: 18–22 mmHg

Answer

PaCO₂ = 20 mmHg
4.Arterial blood gas interpretation

Answer: AGc = 10.75 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 = 139 − (107 + 22) = 10

Correction for albumin

10 + 0.25·(40 − 37) = 10.75 mmol/L

Answer

AGc = 10.75 mmol/L

Still within about 8–12 after correction.

5.Clinical pharmacokinetics

Answer: t½ = 2.5 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.

Elimination rate constant

k = ln(6/1.5)/5 = 1.386/5 = 0.2773 h⁻¹

Half-life

t½ = 0.693/0.2773 = 2.5 h

Answer

t½ = 2.5 h
6.Clinical pharmacokinetics

Answer: D = 334 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.

Creatinine clearance (Cockcroft–Gault)

CrCl = (140 − 72)·90/(72·1.8)·0.85 = 40.14 mL/min

Adjusted dose

D = 1000·40.14/120 = 334.5 ≈ 334 mg

Answer

D = 334 mg

In practice the dose is rounded to a convenient amount and checked against the product information.

7.Clinical nursing

Answer: C · Fever, chills, back pain and low blood pressure

Why this is the answer

An acute haemolytic or febrile reaction can cause fever, rigors, loin or back pain, breathlessness and hypotension. Stop the transfusion and call for help.

Answer

C · Fever, chills, back pain and low blood pressure
8.Clinical nursing

Answer: A · Report it as a near miss through the incident reporting system

Why this is the answer

Near misses are reported so that causes can be studied and systems improved. A just, open culture encourages reporting without blame.

Answer

A · Report it as a near miss through the incident reporting system

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