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

Why must vitamin B12 deficiency be ruled out before treating macrocytic anemia with folic acid alone?

  1. AFolic acid blocks the absorption of all B12 from the diet
  2. BFolic acid makes B12 deficiency anemia much worse within a few days
  3. CFolic acid can fix the anemia while B12-related nerve damage continues
  4. DFolic acid causes iron overload when B12 levels are low
2.Lab values & electrolytesHard

A patient taking an SGLT2 inhibitor has nausea and fast breathing. Glucose 190 mg/dL (10.5 mmol/L), pH 7.15, bicarbonate 10 mEq/L, blood ketones high. What is the most likely diagnosis?

  1. AHyperosmolar hyperglycemic state
  2. BSimple fasting ketosis only
  3. CEuglycemic diabetic ketoacidosis
  4. DRespiratory acidosis from COPD
3.Arterial blood gas interpretationMedium

Practice values: Na⁺ 144 mmol/L, glucose 7 mmol/L and urea 10.8 mmol/L. Calculate the serum osmolality Osm = 2·Na⁺ + glucose + urea to the nearest whole number.

Na⁺ 144 mmol/Lglucose 7 mmol/Lurea 10.8 mmol/L
Osm = mOsm/kg
4.Arterial blood gas interpretationHard

A practice gas shows a metabolic alkalosis with HCO₃⁻ 45 mmol/L. Use expected PaCO₂ = 0.7·HCO₃⁻ + 21 to find the expected PaCO₂.

HCO₃⁻ 45 mmol/L
PaCO₂ = mmHg
5.Clinical pharmacokineticsMedium
VCss​ = 12 mg/LR = ? mg/h · S = 0.8CL = 0.05 L/kg/h87 kg

Practice exercise (made-up values, not clinical guidance). An 87 kg patient is to receive an aminophylline infusion (salt factor S = 0.8: 80% of aminophylline is theophylline). The theophylline clearance is 0.05 L/kg/h. What aminophylline infusion rate R in mg/h gives a steady-state theophylline level of 12 mg/L?

87 kgCL = 0.05 L/kg/hCss = 12 mg/LS = 0.8
R = mg/h
6.Clinical pharmacokineticsHard
t (h)C (mg/L)Css​22 hC = ?

Practice exercise (made-up values, not clinical guidance). A theophylline infusion runs at 30 mg/h with no loading dose. The patient's clearance is 2.5 L/h and the half-life is 9 h. What is the level C after 22 h?

R = 30 mg/hCL = 2.5 L/ht½ = 9 ht = 22 h
C = mg/L
7.Clinical nursingMedium

Which group of features is typical of nephrotic syndrome?

  1. ABlood in the urine, high blood pressure and reduced urine output only
  2. BPolyuria, thirst and weight loss
  3. CHeavy proteinuria, low serum albumin and oedema
  4. DSevere loin pain and fever
8.Clinical nursingHard

A nurse teaches a patient with diabetes about foot care. Which statement shows a need for further teaching?

  1. A"I will check my feet every day for cuts or blisters."
  2. B"I will wear well-fitting shoes."
  3. C"I will see the podiatrist about hard skin."
  4. D"I will walk barefoot at home to toughen my feet."

3Solutions

1.Lab values & electrolytes

Answer: C · Folic acid can fix the anemia while B12-related nerve damage continues

Why this is the answer

Folic acid can improve the blood picture in B12 deficiency but does not prevent nerve damage. Giving folate alone can therefore mask the deficiency while neurological damage progresses. Both levels are checked before treatment.

Answer

C · Folic acid can fix the anemia while B12-related nerve damage continues
2.Lab values & electrolytes

Answer: C · Euglycemic diabetic ketoacidosis

Why this is the answer

SGLT2 inhibitors can cause DKA with only mildly raised glucose because glucose is lost in the urine. Ketones and acidosis must be checked in any unwell patient on these drugs, even with a near-normal glucose.Checked against: Ketones blood test · MedlinePlus (NIH) (2025)

Answer

C · Euglycemic diabetic ketoacidosis
3.Arterial blood gas interpretation

Answer: Osm = 306 mOsm/kg

Derived values: calculated serum osmolality is 2·Na⁺ + glucose + urea with all values in mmol/L (2·Na⁺ + glucose/18 + BUN/2.8 with glucose and BUN in mg/dL); normal is about 275–295 mOsm/kg. The P/F ratio is PaO₂ divided by FiO₂ as a fraction. The alveolar oxygen pressure is PAO₂ = FiO₂·(760 − 47) − PaCO₂/0.8 mmHg at sea level, and the A–a gradient is PAO₂ − PaO₂.

Calculated osmolality

Osm = 2·144 + 7 + 10.8 = 305.8 ≈ 306 mOsm/kg

Answer

Osm = 306 mOsm/kg
4.Arterial blood gas interpretation

Answer: PaCO₂ = 52.5 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

0.7·45 + 21 = 52.5 mmHg

Answer

PaCO₂ = 52.5 mmHg

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

5.Clinical pharmacokinetics

Answer: R = 65.25 mg/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.

Clearance

CL = 0.05·87 = 4.35 L/h

Infusion rate

R = CL·Css/S = 4.35·12/0.8 = 65.25 mg/h

Answer

R = 65.25 mg/h
6.Clinical pharmacokinetics

Answer: C = 9.795 mg/L

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.

Steady-state level

Css = 30/2.5 = 12 mg/L

Rate constant

k = 0.693/9 = 0.07702 h⁻¹

Level after the given time

C = 12·(1 − e−0.07702·22) = 9.795 mg/L

Answer

C = 9.795 mg/L
7.Clinical nursing

Answer: C · Heavy proteinuria, low serum albumin and oedema

Why this is the answer

In nephrotic syndrome, damaged glomeruli leak protein, so albumin falls and fluid moves into tissues, causing oedema. Cholesterol is often raised too.

Answer

C · Heavy proteinuria, low serum albumin and oedema
8.Clinical nursing

Answer: D · "I will walk barefoot at home to toughen my feet."

Why this is the answer

Diabetic neuropathy and poor circulation mean small injuries can become ulcers. Patients should avoid walking barefoot, inspect their feet daily and get professional foot care.

Answer

D · "I will walk barefoot at home to toughen my feet."

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