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Chapter 8: DDx

Stephen J. McPhee; Chuanyi Mark Lu; Diana Nicoll

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    AMA Citation
    McPhee SJ, Mark Lu C, Nicoll D. McPhee S.J., Mark Lu C, Nicoll D McPhee, Stephen J., et al.DDx. In: Nicoll D, Mark Lu C, McPhee SJ. Nicoll D, Mark Lu C, McPhee S.J. Eds. Diana Nicoll, et al.eds. Guide to Diagnostic Tests, 7e New York, NY: McGraw-Hill; . http://accessmedicine.mhmedical.com/content.aspx?bookid=2032&sectionid=152256080. Accessed April 03, 2020.
    MLA Citation
    McPhee SJ, Mark Lu C, Nicoll D. McPhee S.J., Mark Lu C, Nicoll D McPhee, Stephen J., et al.. "DDx." Guide to Diagnostic Tests, 7e Nicoll D, Mark Lu C, McPhee SJ. Nicoll D, Mark Lu C, McPhee S.J. Eds. Diana Nicoll, et al. New York, NY: McGraw-Hill, , http://accessmedicine.mhmedical.com/content.aspx?bookid=2032&sectionid=152256080.

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HOW TO USE THIS SECTION

This section shows how diagnostic tests can be used in differential diagnosis and difficult diagnostic challenges. Material is presented in tabular format, and contents are listed in alphabetical order by disease topic.

Abbreviations used throughout this section include the following:

  • N = Normal

  • Abn = Abnormal

  • Pos = Positive

  • Neg = Negative

  • ↑ = Increased or high

  • ↓ = Decreased or low

  • Occ = Occasional

ACID–BASE DISTURBANCE: LABORATORY CHARACTERISTICS OF PRIMARY OR SINGLE ACID–BASE DISTURBANCE

TABLE 8–1.ACID–BASE DISTURBANCE: LABORATORY CHARACTERISTICS OF PRIMARY OR SINGLE ACID–BASE DISTURBANCE.
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TABLE 8–1. ACID–BASE DISTURBANCE: LABORATORY CHARACTERISTICS OF PRIMARY OR SINGLE ACID–BASE DISTURBANCE.
Disturbance Acute Primary Change Partial Compensatory Response Arterial pH Serum [K+] (meq/L) Anion Gap1 (Unmeasured Anions) (meq/L) Clinical Features
Normal None None 7.35–7.45 3.5–5.0 6–12 None
Respiratory acidosis PCO2 ↑ (CO2 retention) ↑ HCO3– ↓ ↑ N Dyspnea, polypnea, respiratory outflow obstruction,↑ anterior-posterior chest diameter, rales, wheezes. In severe cases, stupor, disorientation, coma.
Respiratory alkalosis PCO2 ↓ (CO2 depletion) ↓ HCO3– ↑ ↓ N or ↓ Anxiety, occasional complaint of breathlessness, frequent sighing, lungs usually clear to examination, positive Chvostek and Trousseau signs.
Metabolic acidosis HCO3– depletion ↓ PCO2 ↓ ↑ or ↓ N or ↑ Weakness, air hunger, Kussmaul respiration, dry skin and mucous membranes, poor skin turgor. In severe cases, coma, hypotension, death.
Metabolic alkalosis HCO3– retention ↑ PCO2 ↑ ↓ N Weakness, positive Chvostek and Trousseau signs, hyporeflexia.

1Anion gap = Na+ – (HCO3– + Cl –) = 6-12. Reference ranges for anion gap may vary based on differing laboratory methods. In hypoalbuminemia, a 2 meq/L decrease in anion gap will occur for every 1 g/dL decline in serum albumin.

Reproduced with permission from Papadakis MA, McPhee SJ, Rabow MW. Current Medical Diagnosis & Treatment 2017. 56th ed. McGraw-Hill, 2017.

ANEMIA: DIAGNOSIS OF COMMON ANEMIAS BASED ON RED CELL INDICES

TABLE 8–2.ANEMIA: DIAGNOSIS OF COMMON ANEMIAS BASED ON RED CELL INDICES.
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TABLE 8–2. ANEMIA: DIAGNOSIS OF COMMON ANEMIAS BASED ON RED CELL INDICES.
Type of Anemia MCV (fL) MCHC (g/dL) Common Causes Common Laboratory Abnormalities Clinical Findings
Microcytic, hypochromic <80 <32 Iron deficiency Hypochromic red cells, elliptocytes, low reticulocyte count, low serum ferritin, low serum iron and absent bone marrow iron, high TIBC, high serum/plasma soluble transferrin receptor (sTfR). Mucositis, brittle nails, bleeding (eg, positive fecal occult blood, menorrhagia), esophageal webs, pica.
Variable, but usually <32 Thalassemias Abnormal red cell morphology, normal to high RBC count, elevated reticulocyte count, normal serum iron parameters, abnormal hemoglobin electrophoresis, high hemoglobin A...

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