NPUAP/NPIAP 2016 classification system — full-thickness tissue depth assessment across anatomical risk sites, from non-blanchable erythema to exposed bone
Stage 1 pressure injury is intact skin with a localized area of non-blanchable erythema, most often over a bony prominence such as the sacrum, heels, or ischial tuberosities. It represents the earliest visible manifestation of localized capillary occlusion from sustained external pressure exceeding capillary closing pressure (normally 32 mmHg arteriolar, 12 mmHg venular). Because the skin barrier is intact, this stage is fully reversible with prompt pressure redistribution — but is also the stage most commonly missed, particularly in patients with darkly pigmented skin.
Pressure injuries develop through a combination of four mechanical forces acting on soft tissue overlying bony prominences:
1. Sustained pressure: • External pressure exceeding mean capillary pressure (~17 mmHg, range 12–32) occludes microvascular flow • Tissue ischemia begins within 1–2 hours of unrelieved pressure >32 mmHg • Reperfusion injury on relief generates additional oxidative damage (ischemia-reperfusion cascade) • Deep tissues (muscle) are MORE sensitive to ischemia than skin — the "pressure cone" concept: maximal damage often occurs at the bone-muscle interface, invisible until it progresses outward
2. Shear force: • Occurs when skeletal structure moves while skin remains fixed against a surface (e.g., patient sliding down in a semi-Fowler bed position) • Distorts blood vessels at an angle, dramatically lowering the pressure threshold needed to occlude flow • Head-of-bed elevation >30° for prolonged periods is a major shear risk factor
3. Friction: • Superficial epidermal damage from surface-to-skin resistance during repositioning/transfers • Rarely causes pressure injury alone but potentiates shear injury
4. Moisture (maceration): • Incontinence-associated dermatitis softens the stratum corneum, lowering mechanical tolerance threshold by up to 25% • Often co-exists with and is misdiagnosed as pressure injury — differential diagnosis is essential (IAD is diffuse/superficial vs. PI is localized over bony prominence)
Stage 1 recognition in darkly pigmented skin: • Traditional "blanch test" (finger pressure, release, observe whitening) is unreliable • Alternative assessment: compare local skin temperature (warmer = inflammation, cooler = deep ischemia), tissue consistency (induration/bogginess), and patient-reported pain/itching at the site • Sub-epidermal moisture (SEM) scanning devices (e.g., Provizio SEM Scanner) measure biocapacitance to detect edema up to 5 days before visible Stage 1 changes — FDA-cleared 2021
At-risk anatomical sites by position: • Supine: sacrum/coccyx, heels, occiput, scapulae, elbows • Lateral (side-lying): greater trochanter, lateral malleolus, lateral condyle of knee, ear • Sitting (wheelchair): ischial tuberosities, sacrum/coccyx, scapulae, posterior heels, popliteal fossa • Prone: chin, chest (breast tissue), knees, toes, symphysis pubis (males), forehead
Stage 2 pressure injury involves partial-thickness loss of the dermis, presenting clinically as a shallow open ulcer with a red-pink, moist wound bed, or as an intact or ruptured serum-filled blister. Critically, adipose tissue and deeper structures are NOT visible, and granulation tissue, slough, and eschar are absent by definition — their presence indicates the wound is actually Stage 3 or deeper and has been mis-staged.
Stage 2 is one of the most frequently mis-staged categories because several non-pressure etiologies mimic its appearance:
Differentiating Stage 2 pressure injury from mimics:
1. Incontinence-associated dermatitis (IAD) / moisture-associated skin damage (MASD): • Distribution: diffuse, follows the pattern of moisture exposure (perineum, buttocks, skin folds) rather than a single bony prominence • Edges: irregular, poorly demarcated vs. pressure injury's well-defined edges over the bony prominence • Often has "copy-lesion" (kissing/mirror-image) pattern on opposing skin folds — pathognomonic for MASD, never seen in pressure injury
2. Skin tears (mechanical/friction injury): • History of trauma (transfer, adhesive removal, minor bump) rather than sustained pressure • Payne-Martin Classification used instead of NPUAP staging • Typically on extremities (forearms, shins) in patients with fragile aged skin, not necessarily over bony prominence
3. Candidiasis: • Satellite lesions, beefy-red with defined scalloped borders, responds to antifungal — not a pressure injury
Blister management: • Intact, tense blister: generally left intact as a physiological dressing (reduces infection risk) unless tense/painful or likely to rupture • If aspirated/deroofed: use sterile technique, preserve the roof as a biologic dressing when possible • Document blister fluid character: serous (Stage 2) vs. serosanguinous/blood-filled (suspect deep tissue injury, NOT Stage 2)
Dressing selection principles for Stage 2: • Goal: maintain moist wound environment without maceration • Thin hydrocolloid or transparent film for low-exudate shallow ulcers • Foam dressing for moderate exudate • Change frequency: hydrocolloid 3–7 days; foam 1–3 days depending on saturation • Avoid wet-to-dry gauze — desiccates the wound bed and causes trauma with each dressing change
Stage 3 pressure injuries involve full-thickness loss of skin in which subcutaneous adipose tissue is visible within the wound bed, and granulation tissue and rolled wound edges (epibole) are frequently present. Depth varies dramatically by anatomical location — the bridge of the nose, ear, occiput, and malleolus have minimal subcutaneous tissue and a Stage 3 injury there can be shallow, whereas the sacrum and buttocks with substantial adipose depth can have extremely deep Stage 3 wounds with undermining and tunneling.
Comprehensive Stage 3 assessment protocol:
1. Wound measurement: • Length × width (greatest perpendicular dimensions, head-to-toe orientation) × depth (deepest point, sterile cotton-tip applicator) • Undermining/tunneling: probe circumferentially using clock-face notation (12 = toward head); document depth at each clock position • Photograph with disposable measuring guide/ruler in frame at each assessment (weekly minimum per most protocols)
2. Tissue composition (visually estimate % of wound bed): • Granulation tissue: beefy red, granular texture — healthy healing tissue • Slough: yellow/tan/white, stringy or mucinous, non-viable — requires debridement • Eschar: black/brown, leathery or hard, non-viable — requires debridement unless stable on ischemic limb/heel • Epibole: rolled, epithelialized wound edges that have "capped" and cannot migrate inward — mechanically or chemically address (silver nitrate, sharp debridement of edge) to re-open the healing edge
3. Debridement method selection: • Sharp/surgical: fastest, for extensive necrotic tissue, requires trained clinician, contraindicated if inadequate blood supply not yet established • Autolytic: body's own enzymes + moist dressing (hydrogel, hydrocolloid); slowest, least painful, contraindicated in infected wounds • Enzymatic: collagenase ointment; selective for necrotic tissue, moderate speed, daily application • Mechanical: wet-to-dry (non-selective, painful, largely obsolete), pulsed lavage, ultrasonic • Biological: sterile maggot (Lucilia sericata) debridement therapy — highly selective, effective for infected/sloughy wounds unresponsive to other methods
4. Periwound assessment: • Maceration (moisture damage), induration (firmness suggesting deep infection), erythema >2cm beyond wound edge (cellulitis), temperature change
5. Infection screening (NERDS/STONEES criteria for superficial vs. deep infection): • NERDS (superficial critical colonization): Non-healing, Exudate increase, Red friable granulation, Debris/slough, Smell • STONEES (deep/surrounding tissue infection): Size increase, Temperature increase, Os (bone) probing/exposed, New areas of breakdown, Exudate increase, Erythema/Edema, Smell — ≥3 STONEES criteria warrants tissue biopsy/culture and possible osteomyelitis workup
Stage 4 represents the deepest classifiable pressure injury: full-thickness skin and tissue loss with exposed or directly palpable fascia, muscle, tendon, ligament, cartilage, or bone. These wounds carry substantial risk of osteomyelitis, sepsis, and prolonged hospitalization, and frequently require surgical consultation for debridement and possible flap reconstruction. Undermining, tunneling, and epibole are common, and depth again varies enormously by anatomical site.
Stage 4 wounds mandate a structured multidisciplinary evaluation given the risk of deep structure involvement:
1. Probe-to-bone (PTB) test: • Sterile blunt probe inserted into wound; hard, gritty sensation at wound base = positive test • Sensitivity ~60%, specificity ~91% for osteomyelitis in diabetic foot literature (extrapolated to PI); positive predictive value increases with elevated ESR/CRP • A positive PTB test in a high-prevalence setting (Stage 4 PI over bony prominence) substantially raises pretest probability of osteomyelitis
2. Imaging: • Plain radiograph: first-line, low sensitivity early (~60%), findings lag clinical infection by 2+ weeks • MRI with contrast: gold standard for soft tissue/bone marrow edema characterization; sensitivity 90%, specificity 79% • CT: alternative when MRI contraindicated (pacemaker); better cortical bone detail • Bone scan (99mTc): high sensitivity but low specificity, especially with adjacent soft tissue inflammation
3. Definitive diagnosis: • Bone biopsy (surgical or percutaneous) for histology AND culture is the diagnostic gold standard • Culture guides targeted antibiotic therapy — avoid empiric broad-spectrum therapy without a specimen when feasible • Common organisms: Staphylococcus aureus (including MRSA), Enterococcus, Pseudomonas aeruginosa, and polymicrobial anaerobic flora given proximity to perineum
4. Multidisciplinary management: • Infectious disease: pathogen-directed antibiotics, typically 6 weeks IV for confirmed osteomyelitis (per IDSA-adapted protocols), shorter with adequate surgical debridement • Plastic/reconstructive surgery: myocutaneous flap closure (gluteal, tensor fasciae latae, gracilis flaps depending on site) once wound bed is clean and patient medically optimized • Nutrition: protein 1.25–1.5 g/kg/day, arginine/glutamine/HMB-supplemented formulas (e.g., Juven) shown to accelerate closure in RCTs • Nursing: strict pressure redistribution — Group 2/3 support surface (low-air-loss or alternating pressure), turning schedule individualized (not rigid q2h if not tolerated hemodynamically), incontinence management
5. When surgery is not an option: • Medically fragile/end-of-life patients: focus shifts to symptom control, odor management (topical metronidazole gel), exudate management, and comfort — "palliative wound care" is an accepted, guideline-endorsed approach, not a failure of care
A positive probe-to-bone test combined with an exposed, visible osseous surface in a Stage 4 pressure injury should prompt bone biopsy for culture and histology before committing to a prolonged empiric antibiotic course — misdiagnosis and unnecessary prolonged antibiotics contribute significantly to resistance and C. difficile risk in this population.
An unstageable pressure injury is full-thickness skin and tissue loss in which the true depth of tissue damage cannot be confirmed because the wound bed is obscured by slough (yellow, tan, gray, green, or brown) and/or eschar (tan, brown, or black). Until enough slough/eschar is removed to expose the base of the wound, the true stage (3 or 4) cannot be determined. A critical exception exists: stable, dry, adherent, intact eschar without erythema or fluctuance on an ischemic limb or the heel should NOT be debrided — it functions as the body's natural biological cover.
The unstageable category exists specifically because staging requires visualization of the deepest viable structure in the wound bed — an obscured base cannot be accurately staged, and premature debridement in the wrong clinical context can convert a stable, protected wound into a limb-threatening one.
Step 1 — Assess vascular supply to the limb: • Palpate posterior tibial and dorsalis pedis pulses; if non-palpable, obtain Ankle-Brachial Index (ABI) • ABI <0.5 or non-compressible vessels (common in diabetics, calcified vessels) → high risk; refer to vascular surgery BEFORE any debridement • Adequate perfusion (ABI 0.8–1.3, palpable pulses, capillary refill <3 sec) → safe to proceed with debridement per standard protocol
Step 2 — Characterize the eschar: • Stable eschar criteria (ALL must be present to justify non-removal): dry, hard, adherent firmly to wound margins, intact (no fluctuance/drainage), no periwound erythema, no odor, no pain on palpation • If ANY of the following present, urgent debridement indicated regardless of location: fluctuance (suggests underlying abscess), periwound erythema/warmth (suggests cellulitis/deep infection), purulent drainage or malodor, crepitus (suggests gas-forming infection — surgical emergency), sepsis physiology
Step 3 — Heel-specific consideration: • Heel pressure injuries with stable dry eschar and adequate arterial flow are managed with "watchful waiting" — offloading (heel suspension boots/pillows), serial monitoring, NO debridement • Rationale: heel has minimal soft tissue coverage over calcaneus; premature debridement in a limb with borderline perfusion risks non-healing wound, osteomyelitis, or amputation • If eschar becomes unstable (any Step 2 red flag develops) → immediate debridement regardless of prior "watch" status
Step 4 — Debridement approach once indicated: • Sharp conservative debridement at bedside for loosely adherent slough • Enzymatic (collagenase) for firmly adherent slough when sharp debridement is not immediately available/appropriate • Cross-hatching of stable eschar (creating fissures without full removal) sometimes used to assess depth without committing to full excision — controversial, institution-dependent
Documentation requirement: • Chart explicitly as "Unstageable — obscured by [slough/eschar]" — never guess or assign a numeric stage to an obscured wound • Once debrided and base visualized, re-stage at that encounter using standard NPUAP criteria (will always be Stage 3 or Stage 4, never lower, since full-thickness loss is already established)
Deep tissue pressure injury (DTPI) presents as intact or non-intact skin with a localized area of persistent non-blanchable deep red, maroon, or purple discoloration, or as an epidermal separation revealing a dark wound bed or a blood-filled blister. It results from intense and/or prolonged pressure and shear forces at the bone-muscle interface — the deepest tissue layers, which are more metabolically sensitive to ischemia than the skin above them. The visible surface discoloration often dramatically underrepresents the extent of underlying tissue necrosis, and DTPI can evolve rapidly (within 24–72 hours) to reveal a full-thickness Stage 3 or Stage 4 wound despite optimal treatment.
DTPI is fundamentally different from Stage 1 (superficial epidermal ischemia) — it is a manifestation of damage that began in the deep tissue and is only now becoming visible at the surface.
The pressure cone concept: • When soft tissue is compressed between a bony prominence and an external support surface, pressure is NOT distributed evenly through the tissue depth • Pressure is highest immediately adjacent to the bone and decreases toward the skin surface — visualized as an inverted cone, apex at bone, base at skin • Muscle tissue has a higher metabolic rate and oxygen demand than skin/subcutaneous fat, making it substantially more vulnerable to ischemic injury at a given pressure/duration • Consequence: by the time discoloration is visible at the skin surface, extensive muscle necrosis may already be present at the bone interface — the visible lesion is the "tip of the iceberg"
Differentiating DTPI from mimics:
1. vs. bruise/ecchymosis (trauma): • DTPI occurs specifically over a bony prominence exposed to sustained pressure (not random trauma sites) • History consistent with immobility, surgery (prolonged OR table time is a classic DTPI risk period), or device-related pressure • No history of a discrete traumatic event
2. vs. Stage 1 pressure injury: • DTPI discoloration is deep red/maroon/purple — darker and more intense than Stage 1's pink-red erythema • DTPI often has a firm/boggy or mushy texture change and may be warmer or cooler than surrounding tissue • A blood-filled blister strongly favors DTPI over a Stage 2 (which has a clear/serous-filled blister)
3. vs. deep vein thrombosis or vasculitis: • Localization strictly over/adjacent to a bony prominence favors DTPI; diffuse limb involvement favors DVT; consider Doppler ultrasound if diagnosis unclear
Clinical trajectory and documentation: • Once identified, DO NOT downstage as it evolves — if a DTPI becomes an open wound, it is documented as "DTPI, now with skin loss — evolving," and once the depth is fully revealed, re-staged per standard NPUAP criteria (typically Stage 3 or 4) • Serial photography every 24–48 hours during the acute evolution period is recommended to track progression objectively • Prognosis is guarded: a meaningful proportion of DTPI lesions progress to full-thickness injury despite maximal pressure redistribution, because the deep tissue damage was already established at the time of first visible discoloration
Because DTPI represents damage that has already occurred deep in the tissue before it becomes visible at the skin, the single most important intervention is prevention: immediate and complete pressure offloading of the affected site the moment discoloration is noted, combined with an urgent, comprehensive reassessment of the patient's entire pressure redistribution plan (surface, turning schedule, moisture, nutrition, and medical devices).