Read the pH: below 7.35 is acidosis, above 7.45 alkalosis.
Find the value that explains it: PaCO₂ moving opposite to the pH is respiratory, HCO₃⁻ moving with the pH is metabolic.
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.
In a metabolic acidosis, compare PaCO₂ with Winter’s formula, 1.5·HCO₃⁻ + 8 ± 2, and calculate the anion gap Na⁺ − (Cl⁻ + HCO₃⁻).
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
Pick the relation: C = C₀·e−kt, LD = Vd·C/(S·F), maintenance rate = CL·Css/(S·F), τ = ln(Cpeak/Ctrough)/k.
For kidney function, estimate CrCl with Cockcroft–Gault first, then scale the dose or the interval.
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?
AOnly after the baby is born
BAt about 24–28 weeks of pregnancy
CAt about 6–8 weeks of pregnancy
DAt about 38–40 weeks of pregnancy
2.Lab values & electrolytesHard
A patient on a thiazide diuretic has a sodium of 129 mEq/L, a potassium of 3.2 mEq/L and feels weak. Which explanation fits best?
AThe patient has primary hyperaldosteronism with high sodium
BThe diuretic is causing sodium and potassium loss
CThe patient is overhydrated from IV fluids only
DThe results are typical of kidney failure
3.Arterial blood gas interpretationMedium
A practice arterial blood gas shows pH 7.36, PaCO₂ 33 mmHg (4.4 kPa) and HCO₃⁻ 18 mmol/L. The primary disorder is metabolic acidosis. How far is it compensated? Click your answer.
pH 7.36PaCO₂ 33 mmHg (4.4 kPa)HCO₃⁻ 18 mmol/L
1uncompensated
2partially compensated
3fully compensated
4.Arterial blood gas interpretationHard
Practice values: Na⁺ 132, Cl⁻ 94 and HCO₃⁻ 17 mmol/L. Calculate the delta ratio (AG − 12)/(24 − HCO₃⁻), using AG = Na⁺ − (Cl⁻ + HCO₃⁻). Give two decimals.
Na⁺ 132 mmol/LCl⁻ 94 mmol/LHCO₃⁻ 17 mmol/L
delta ratio =
5.Clinical pharmacokineticsMedium
Practice exercise (made-up values, not clinical guidance). After an IV dose of vancomycin, the level is 30 mg/L at 1 h and 11.9 mg/L at 9 h. Assume first-order elimination. Find the elimination rate constant k.
C1 = 30 mg/L (1 h)C2 = 11.9 mg/L (9 h)
k = h⁻¹
6.Clinical pharmacokineticsHard
Practice exercise (made-up values, not clinical guidance). A patient's gentamicin half-life is 2.5 h. The target is a peak of 10 mg/L and a trough of 0.5 mg/L. Assuming rapid IV doses and first-order elimination, what dosing interval τ lets the level fall from the peak to the trough? Round to one decimal place.
t½ = 2.5 hCpeak = 10 mg/LCtrough = 0.5 mg/L
τ = h
7.Clinical nursingMedium
Why is serum lactate measured in a patient with suspected sepsis?
AIt shows whether the patient has a urinary tract infection
BIt measures the patient's blood glucose
CIt confirms which antibiotic to give
DA raised lactate indicates poor tissue perfusion and higher risk
8.Clinical nursingHard
A patient with known cirrhosis suddenly vomits large amounts of bright red blood and becomes tachycardic. What is the most likely cause?
ABleeding from ruptured oesophageal varices
BA lower GI bleed from diverticular disease
CA Mallory-Weiss tear from a small amount of vomiting
DSwallowed blood from a nosebleed
3Solutions
1.Lab values & electrolytes
Answer: B · 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
B · At about 24–28 weeks of pregnancy
2.Lab values & electrolytes
Answer: B · The diuretic is causing sodium and potassium loss
Why this is the answer
Thiazide diuretics commonly cause hyponatremia and hypokalemia, especially in older adults. The nurse reports the results, and the prescriber reviews the drug and electrolyte replacement.Checked against:Thiazide Diuretics · StatPearls (NCBI Bookshelf) (2025)
Answer
B · The diuretic is causing sodium and potassium loss
3.Arterial blood gas interpretation
Answer: fully compensated
Reading a blood gas (ROME): first the pH: below 7.35 is acidaemia, above 7.45 alkalaemia. Then find the value that explains it. PaCO₂ (normal 35–45 mmHg, 4.7–6.0 kPa) is the respiratory part and moves Opposite to the pH in a respiratory disorder; HCO₃⁻ (normal 22–26 mmol/L) is the metabolic part and moves in the same direction as the pH (Respiratory Opposite, Metabolic Equal). The other value shows the compensation: normal means uncompensated; abnormal in the direction that pulls the pH back while the pH is still abnormal means partially compensated; a normal pH with both values abnormal means fully compensated. Ranges differ slightly between laboratories; these are practice values.
ROME, value by value
pH 7.36: within 7.35–7.45
PaCO₂ 33 mmHg: low (35–45)
HCO₃⁻ 18 mmol/L: low (22–26)
Compensation
fully compensated
Answer
fully compensated
4.Arterial blood gas interpretation
Answer: delta ratio = 1.286
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 = 132 − (94 + 17) = 21
Delta ratio
(21 − 12)/(24 − 17) = 9/7 = 1.286
Answer
delta ratio = 1.286
Between 1 and 2: a pure high anion gap acidosis.
5.Clinical pharmacokinetics
Answer: k = 0.1156 h⁻¹
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(30/11.9)/8 = 0.9247/8 = 0.1156 h⁻¹
Answer
k = 0.1156 h⁻¹
6.Clinical pharmacokinetics
Answer: τ = 10.8 h
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.
Rate constant
k = 0.693/2.5 = 0.2773 h⁻¹
Dosing interval
τ = ln(10/0.5)/0.2773 = 2.996/0.2773 = 10.8 h
Answer
τ = 10.8 h
In practice the interval is rounded to a convenient value such as 8, 12, 24 or 36 h, and the levels are checked again.
7.Clinical nursing
Answer: D · A raised lactate indicates poor tissue perfusion and higher risk
Why this is the answer
Lactate rises when tissues are poorly perfused. A high lactate in sepsis signals greater severity and is used to guide fluid resuscitation.
Answer
D · A raised lactate indicates poor tissue perfusion and higher risk
8.Clinical nursing
Answer: A · Bleeding from ruptured oesophageal varices
Why this is the answer
Portal hypertension in cirrhosis causes oesophageal varices, which can bleed massively. This needs resuscitation, urgent endoscopy and specific drug treatment.