What are the key characteristics to report when describing an unknown rash for medical evaluation?

How to Describe a Rash for Medical Evaluation

When describing an unknown rash, systematically document morphology, distribution, timing relative to fever, associated symptoms, and specific high-risk features—particularly involvement of palms/soles, non-blanching quality, and presence of systemic toxicity—as these characteristics distinguish life-threatening conditions from benign eruptions. 1, 2

Essential Morphologic Characteristics to Document

Primary Lesion Type

• Describe whether lesions are macular (flat), papular (raised), vesicular (fluid-filled), petechial (pinpoint non-blanching), or purpuric (larger non-blanching), as this classification guides the differential diagnosis and determines urgency 3, 4
• Specify if the rash blanches with pressure, since non-blanching rashes represent medical emergencies requiring antibiotics within 1 hour, while blanching rashes typically indicate less urgent conditions 2
• Note the size, shape, and color of individual lesions, including whether they are pink, red, violet, or have central changes like umbilication 3, 5
• Document the presence of scale, crusting, weeping, or erosions, as these features suggest secondary infection or specific dermatologic conditions 3

Distribution Pattern and Body Surface Area

• Record the percentage of body surface area involved, as this determines severity grading and influences management decisions 1
• Specifically note involvement of palms and soles, which suggests serious rickettsial or bacterial infection requiring immediate treatment 3, 6, 2
• Document whether the pattern is centripetal (starting peripherally and spreading centrally) or centrifugal, as Rocky Mountain Spotted Fever characteristically begins on ankles, wrists, and forearms before spreading 3, 2
• Identify areas of sparing or concentration, such as flexural surfaces in atopic dermatitis, extensor surfaces in erythema nodosum, or perineal accentuation in Kawasaki disease 3, 1, 5
• Note involvement of face, mucous membranes, or genitalia, as mucosal involvement suggests Stevens-Johnson Syndrome or other severe drug reactions 1

Critical Temporal Relationships

Timing Relative to Fever

• Document whether rash appeared before, during, or after fever onset, as roseola characteristically presents with rash appearing when fever breaks after 3-4 days, while Rocky Mountain Spotted Fever rash appears 2-4 days after fever onset 3, 6, 7
• Record the duration of fever before rash development, since this timing distinguishes between viral exanthems, drug reactions (typically 6 weeks after drug exposure for DRESS), and bacterial infections 1, 8
• Note if the rash is evolving or static, as Rocky Mountain Spotted Fever progresses from blanching pink macules to maculopapular with petechiae to generalized petechial rash over 5-6 days 3, 2

Associated Symptoms and Systemic Features

High-Risk Clinical Features

• Document presence of fever, altered mental status, hypotension, or other signs of systemic toxicity, as these require immediate hospitalization regardless of rash appearance 6, 2
• Record symptoms of meningoencephalitis including severe headache, focal neurologic deficits, or sudden deafness, which can occur with Rocky Mountain Spotted Fever or ehrlichiosis 3
• Note presence of pruritus versus pain, as pruritic lesions suggest atopic dermatitis, pityriasis rosea, or viral exanthems, while painful lesions suggest pyoderma gangrenosum or Sweet's syndrome 3, 7
• Document lymphadenopathy, which is usually secondary to extensive skin disease but may cause alarm 3

Exposure History and Medications

• Obtain complete medication history including all drugs taken in the preceding 5-28 days, as drug reactions are a critical differential diagnosis for diffuse rashes 1
• Record tick exposure history and recent travel, though absence of tick exposure does not exclude Rocky Mountain Spotted Fever (present in only 60% of cases) 3, 2
• Document environmental exposures, contact with ill persons or animals, as the patient's dog dying with similar symptoms in one case led to diagnosis of Rocky Mountain Spotted Fever 3, 8

Laboratory and Clinical Findings to Report

Routine Laboratory Abnormalities

• Include complete blood count results showing leukopenia, thrombocytopenia, or anemia, as these findings are characteristic of ehrlichiosis and help distinguish tickborne rickettsial diseases from viral syndromes 3
• Report ferritin levels if available, since iron deficiency is a common cause of generalized pruritus and responds to iron replacement 3

Dermatologic Signs

• Test and document whether lesions demonstrate the Koebner phenomenon (lesions appearing at sites of trauma), as this occurs with pyoderma gangrenosum and certain other conditions 3, 5
• Note presence of pathergy (lesions preceded by trauma), which is characteristic of pyoderma gangrenosum 3

Common Pitfalls to Avoid

• Do not wait for the classic triad of fever, rash, and tick bite in Rocky Mountain Spotted Fever, as it is present in only a minority at initial presentation, yet mortality reaches 5-10% if untreated 1, 2
• Do not exclude serious disease based on absence of rash initially, as up to 20% of Rocky Mountain Spotted Fever cases have absent or atypical rash, and rash in ehrlichiosis may not appear until 5 days after onset 3
• Do not assume a blanching rash is benign, as meningococcal disease may start with blanching rash before rapidly progressing to non-blanching petechiae and purpura 2
• Do not rely solely on rash appearance to distinguish Rocky Mountain Spotted Fever from ehrlichiosis in children, as both can present with similar maculopapular or petechial patterns 3